Emission Inspections of Vehicles in Operation—Case Study for Slovakia

Abstract

Air pollution poses a serious threat to human health and the environment. Emissions from motor vehicles, especially in large cities, contribute significantly to this problem. This study analyzes the results of emission inspections in the Slovak Republic to identify factors influencing emissions and their impact on air quality. The research analyzed data from emission inspections and their relationship to vehicle age, fuel type, and type of failure. The results show that older vehicles, especially those aged 10 to 20 years, have a higher probability of failing to meet emission standards. Specifically, up to 42.75% of diesel vehicles aged 15 to 20 years were rated as unfit, compared to 33.07% of gasoline vehicles in the same age category. An increased proportion of unfit vehicles was recorded for diesel engines, which indicates their negative impact on air quality. The most common failures were related to direct emission measurements. These findings have implications for environmental policy and the regulation of vehicle imports to improve air quality and reduce pollution. Data on emission inspections were drawn from the national system and show knowledge about the observation of emission inspections carried out during one calendar year. The study recommends the introduction of stricter control mechanisms for older vehicles, supporting the renewal of the vehicle fleet, and the implementation of modern technologies to reduce emissions. Rigorous emission inspections are essential for the protection of public health. Regular inspections and modern technologies reduce emissions of harmful substances, thus contributing to the improvement of air quality and public health.

1. Introduction

In recent decades, increasing urbanization, development of transport, and industrial activity have significantly contributed to air pollution, which poses a serious threat to human health and the environment [1,2]. Emissions from motor vehicles are among the most important sources of pollution, especially in large urban agglomerations. For this reason, in many countries, more and more emphasis is placed on the regulation and control of emission limits, which are set by legislation [3,4]. Šarkan et al., in their research, point to the importance of emission inspections, which have become an important tool to ensure the compliance of vehicles with these limits, thereby reducing the amount of harmful substances released into the air [5].

Emission inspections not only contribute to the protection of the environment, but also have a significant impact on public health, as air pollution can lead to various health problems, such as respiratory diseases, cardiovascular problems, or an increased risk of premature deaths [6,7,8]. According to several studies, systematic emission controls significantly reduce emissions of carbon monoxide (CO), nitrogen oxide (NO_X), and hydrocarbons (HC) from motor vehicles [9,10]. These substances are among the most harmful components of traffic pollution and have a direct impact on air quality and public health. The results of a study carried out in cities with strict emission standards showed that after the introduction of regular controls, NO_X concentrations decreased by more than 20% and CO by 15% [11]. Similarly, other research that analyzed the results of emission inspections in various European countries found that the effectiveness of these measures also depends on the consistent enforcement of legal requirements and technological innovations in the field of motor vehicles [12,13].

In the past, motor vehicles had a significant share in global air pollution, with their share estimated at 4 to 40% depending on the type of pollutant and specific location [14,15]. The main pollutants emitted by combustion engine vehicles of air quality concern include nitrogen oxides NO_X, particulate matter PM, hydrocarbons, CO, and various trace amounts of other substances. These emissions are generally referred to as pollutants or exhaust emissions. In response to this problem, governments and industry have been taking steps to reduce exhaust emissions from road vehicles for decades. As a result, modern vehicles today contribute only minimally to air pollution in many places [16]. Thanks to technologies such as catalytic converters for treating gaseous emissions and particulate filters, modern internal combustion engines can produce very low levels of all major pollutants [17]. Some vehicles achieve emissions that are practically unmeasurable even when using the most accurate instruments [18]. This progress was made possible by strict global standards that regulate PM emissions to 4.5 mg/km (Euro 6d), 3 mg/km (China 6), and 3 mg/mile (Tier 3, USA) and NO_X emissions to 60 mg /km (Euro 6d), 35 mg/km (China 6), and 20 mg/mile (Tier 3).

In the past, however, it was not common for vehicles in real operation to meet certification values. The “Dieselgate” affair [19] and also other research [20] drew attention to this problem. In 2017, European and 2020 Chinese legislation introduced the requirement to measure real driving emissions (RDE) during vehicle certification. In these tests, emissions are measured while driving on roads using a portable emission measurement system (PEMS), while the results take into account the variables of real traffic conditions [21,22]. Tests include driving in the city, in the countryside, and on the highway. In addition to certification tests, independent providers perform tests in a wider range of conditions, and the results are publicly available [23]. Emission values expressed in g/km, whether from certification or RDE, represent averages and do not reflect continuous emissions during driving. Actual emissions depend on engine speed, load, driving style and temperature of aftertreatment systems. A significant part of the emissions can be released during the cold start phase when the engine and systems have not yet reached the optimal temperature [24,25].

In the Slovak Republic, the system for evaluating road vehicle emissions is based on emission inspection stations, where emissions are checked when the vehicle is stationary with the help of an exhaust gas analyzer. Vehicles are required to undergo regular emission inspections. During the inspection, the vehicle is checked to see if its operation does not cause excessive air pollution. The performance of the emission inspection depends on the type of vehicle driven. Currently, the emission inspection is divided between vehicles with gasoline engines and vehicles with diesel engines. The production of emissions from gasoline vehicles is controlled at idling speed (rpm) and at increased speed (rpm). When directly measuring exhaust emissions, only smoke is measured in vehicles with a diesel engine. In such a case, the technician cannot assess the condition of systems such as the DPF filter, AdBlue, catalytic converter, etc. In such a case, however, a legislative change is necessary so that particulate emissions can be measured, such as in Germany or Belgium. The same applies to gasoline engines. This is not a problem only in the Slovak Republic but also in other countries. In this respect, the research is unique because it presents a comprehensive overview of emission controls and vehicle imports. The same conditions apply to alternative fuels. In general, during the emission inspection of a gasoline-powered vehicle, it is possible to define different emission systems:

• Without catalyst: controlled parameters: engine temperature, engine speed, CO production, HC production;
• Uncontrolled catalyst: controlled parameters: engine temperature, engine speed, CO production, HC production;
• Controlled catalyst: controlled parameters: engine temperature, engine speed, CO production, HC production;
• Controlled catalytic converter with OBD (on-board diagnostic): controlled parameters: engine temperature, engine speed, CO production, HC production, lambda value.

For vehicles with a diesel engine, the check is unified for all types of engines (without catalytic converter, uncontrolled catalytic converter, uncontrolled catalytic converter with OBD). From the point of view of emission production, only the smokiness of the vehicle is controlled. This means that the content of individual components of harmful substances in exhaust gases is not analyzed.

The limit values monitored at emission inspection stations are strictly determined by the vehicle manufacturer for the specific type of engine identified in the vehicle; they are listed mainly in the manufacturer’s service information documents or in specialized specialist catalogs [26]. Currently, the control is that the success of emission inspections is only a matter of technical measures and proper implementation and control. Studies show that countries with advanced control and revision systems perform better in reducing emissions, while problems with high pollution levels persist where controls are less stringent or ineffective [27,28,29].

2. Motivation for Research

Air pollution significantly affects human health and the environment. In Europe, limits for nitrogen dioxide (NO₂) and particulate matter (PM) are often exceeded, especially in cities with dense traffic. The increase in the number of passenger cars can be presented by the degree of motorization, that is, the number of passenger cars per inhabitant of Slovakia. While in 1995, there was one passenger car per more than five inhabitants, in 2014 it was already 2.8 inhabitants [30]. In this case, we can talk about a rapid increase in the number of passenger cars registered in the Slovak Republic. In 2017, road transport in the EU contributed to 39% of nitrogen oxide (NOx) emissions and 11% of particulate matter emissions [31]. The Dieselgate affair contributed to this finding. Until 2017, the NEDC test cycle was used to measure fuel consumption and vehicle emissions in Europe, including Slovakia. However, this cycle was designed in the 1990s and corresponded to the traffic conditions of the time—it was not very dynamic; it was short and carried out in a laboratory and did not take into account real driving conditions, such as higher speeds, frequent acceleration, air conditioning, or various vehicle equipment. As a result, real-world emissions and fuel consumption in traffic were often significantly higher than the official figures [32]. This difference was dramatically demonstrated during the Dieselgate emissions scandal in 2015, when it was revealed that several car manufacturers had used software that recognized a laboratory test and temporarily adjusted the engine to make the vehicle comply with emission limits. However, in regular traffic, these vehicles emit many times more nitrogen oxides (NOx), causing serious health and environmental problems, particularly in urban areas. In response to this scandal, a new globally harmonized cycle, the WLTP (Worldwide Harmonized Light Vehicles Test Procedure), was introduced, which is longer, more dynamic, and better reflects real-world driving conditions. In addition, RDE (real driving emissions) was also introduced, which measures emissions directly while driving on regular roads using portable measurement systems (PEMSs). These changes were intended to prevent cheating and increase the credibility of official data. In Slovakia, as in the entire EU, the transition to WLTP and RDE tests meant

• Increasing the reliability of data on the consumption and emissions of new vehicles;
• More precise calculation of registration fees and taxes linked to CO₂ emissions;
• Pressure to replace the vehicle fleet, as older vehicles without RDE systems are significantly lagging behind in terms of emissions.

The Slovak Republic has long been one of the countries with one of the oldest vehicle fleets in the EU. The average age of a vehicle here exceeds 13 years, while many vehicles still only meet Euro 3 or Euro 4 emission standards. These vehicles often significantly exceed the limits for NOx and particulate emissions, which is a problem, especially in large cities. Although Slovakia is participating in European measures, the replacement of old vehicles with newer ones with WLTP and RDE certification is slow. The reasons are economic factors and, above all, insufficient motivation to scrap old vehicles. There is a lack of more extensive mechanisms to support vehicle renewal, as well as tax incentives for low-emission vehicles.

When examining the amount of emissions produced by road vehicles, it is necessary to focus on the cause of increased production. In many cases, this is caused by the failure of a component that directly participates in the preparation of the fuel mixture in the combustion engine. Often, the driver does not have information about such a malfunction. Many conclusions of research work prove that damage or malfunction of a component can cause increased production of emissions [33]. Dziubak et al., in their study, investigated the effect of air filter pressure drop on the composition and changes in diesel combustion engine emissions [34]. The results show the influence of the flow resistance of the air filter of a modern car engine on its performance, especially on changes in the emissions of individual exhaust gas components. Another study evaluated exhaust emissions from a vehicle engine operating in dynamic states and conditions corresponding to real driving. The results of the study by Andrych-Zalewská et al. point to the sensitivity of exhaust emissions to dynamic states differentiated according to the type of dynamic states, test, and measured substance. In general, the sensitivity of exhaust emissions to dynamic conditions was greater for torque changes than for speed changes [35].

The results of the research provide valuable information in terms of vehicle imports as well as the emission status of passenger vehicles up to 3.5 t intended for the transport of people in the territory of the Slovak Republic. Slovakia, as part of the European Union, is becoming a “collector” of old cars from parts of Europe, and the operation of old vehicles represents an economic burden for their owners. The results of the contribution are intended to point out this case as well as the poor situation in the results of emission inspections.

3. Import of Vehicles from Abroad

In recent years, in Slovakia, as in other countries in Eastern Europe, the import of used vehicles from abroad has become a relatively widespread trend, with a significant proportion of these vehicles coming from European Union countries, such as Germany, Italy, France, and Austria. This phenomenon is mainly due to the affordability of used vehicles compared to new cars, which are financially unaffordable for many consumers. As a result, the average age of imported cars is increasing, with a significant proportion of imported vehicles exceeding 10 years. Several studies point to the impact of used vehicle imports on new vehicle sales [36,37]. The following graph shows how newly registered vehicles are included in the national vehicle registry. For a more comprehensive presentation, percentage values are presented from the total number of registered vehicles of 2,571,631 units.

The graphic processing in Figure 1 shows the analysis of vehicles registered in the Slovak Republic. The horizontal axis shows different categories of vehicle registration, such as “individual import”, “migration”, “new vehicle”, and “change of technical data of the registered vehicle”. The vertical axis expresses the percentage share of individual categories from the total number of registered vehicles. The highest share, 59.52%, is made up of new vehicles, which indicates that the majority of vehicles registered in Slovakia are newly manufactured cars. This share significantly prevails over other categories. The second highest share is represented by individual imports, which reach 36.27%. This value indicates that although new vehicles are dominant, a significant number of registered vehicles originate from abroad through individual imports. Migration, i.e., the movement of vehicles between regions or countries, accounts for only 0.48% of total registrations, a negligible share compared to the previous two categories. The change in the technical data record of a registered vehicle represents 3.73% of the total number of registrations. Although this share is significantly lower than the share of new vehicles and individual imports, it is still significantly higher than the share of migration and other specific categories.

In the next graph, it is possible to follow the method of registration of vehicles in the territory of the Slovak Republic during the observed period. The period between 2018 and 2021 was chosen as the monitored period. In the selected period, 538,876 passenger vehicles intended for the transport of people were registered in the Slovak Republic. The graph shows a percentage expression for better visualization of the data.

The graph (Figure 2) shows the method of vehicle registration in the monitored period, while analyzing the percentage share of different registration categories in 2018, 2019, 2020, and 2021. Individual years are shown on the horizontal axis, and percentage values representing the shares of individual registration methods are shown on the vertical axis.

Within the monitored years, the most significant shares were recorded in two categories:

• Individual imports;
• Registration of new vehicles.

In 2018, the share of new vehicles was the highest, reaching 15.19%, while individual imports represented 12.04%. A year later, in 2019, both categories increased, with the share of new vehicles reaching 15.69% and individual imports rising slightly to 12.14%.

In 2020, the share of new vehicles decreased to 11.57%, which was accompanied by a decrease in individual imports to 9.83%. This trend continued in 2021, when a further decrease in the share of new vehicles was recorded to 11.70%, and individual imports reached 10.53%. It is clear from the graph that the share of new vehicles decreased but still remained dominant compared to individual imports. This phenomenon of the decrease in vehicle registration could have been caused by the global coronavirus pandemic.

Other categories, such as migration or changing the technical data record of a registered vehicle, have relatively low shares compared to the main categories. Migration hovered within a few hundredths of a percent, reaching 0.03% and 0.04% in 2018 and 2019 and rising slightly to 0.04% and 0.05% in 2020 and 2021, respectively. Overall, the graph shows that the dominant methods of vehicle registration in Slovakia are new vehicle registrations and individual imports, while the share of these categories fluctuated slightly over the years.

Older vehicles, which are often manufactured to older emission standards (e.g., Euro 3, Euro 4), generally have higher exhaust emissions compared to newer vehicles that meet more stringent emission standards (e.g., Euro 5, Euro 6). This means that even if the vehicle imported into the country is in technically good condition, its emission characteristics may be significantly worse compared to newer models. In the following graph, it is possible to follow the analysis of the age of imported vehicles during the years 2018 to 2021. The figure is also supplemented by

Table 1. Analysis of the age of imported vehicles in the monitored period (all types of fuel).

All Types of Fuels	Up to 5 Years	From 5 to 10 Years	From 10 to 15 Years	From 15 to 20 Years	From 20 to 25 Years	Over 25 Years
2018	10.02%	7.40%	7.05%	2.16%	0.24%	0.16%
2019	10.41%	6.30%	7.94%	2.15%	0.28%	0.17%
2020	9.52%	4.94%	5.27%	1.82%	0.31%	0.19%
2021	10.00%	5.38%	5.44%	2.17%	0.43%	0.23%

, which presents the source data. As part of the investigation, it concerned vehicles that were registered in the vehicle register for the purpose of individual importation. The total number of these vehicles is 240,000; for better data processing and presentation, only % percentages of the total number are presented graphically.

The graph as well as the table analyzes the age of imported vehicles in the observed period, namely from 2018 to 2021. The horizontal axis shows individual years, while the vertical axis shows the percentage shares of vehicles in different age categories, such as “up to 5 years”, “from 5 to 10 years”, “from 10 to 15 years”, “from 15 to 20 years”, “from 20 to 25 years” and “over 25 years”. The largest share of imported vehicles in all the monitored years is made up of vehicles in the “under 5 years” age category, which indicates that the most frequently imported cars are relatively new. In 2018, the share of these vehicles was 10.02%; in 2019, it reached 10.41%; in 2020, it fell slightly to 9.52%, and in 2021, it stabilized at 10.00%. This trend shows a stable interest in newer vehicles with minimal variations in individual years. The second most numerous category is vehicles aged “from 5 to 10 years”. In 2018, they made up 7.40%; in 2019, they increased to 7.94%; in 2020, their share fell to 4.94%; and in 2021, they increased to 5.38%. Although the share of this category has changed over the years, it still represents a significant part of imports. The share of vehicles in the age category “from 10 to 15 years” was relatively similar in 2018 and 2019, with values of 7.05% and 6.30%. In 2020, this share dropped to 5.27%, and in 2021, it increased slightly to 5.44%. However, this decline in the middle years still indicates that even older vehicles, but less than 15 years old, make up an important part of imports. Other age categories, such as “from 15 to 20 years old”, “from 20 to 25 years old”, and “over 25 years old”, have a low share of total imports, with their shares below 2.5% in all years. The category “from 20 to 25 years old” and “over 25 years old” reaches the lowest values, which is below 0.5% in all the monitored years. Overall, the graph shows that the Slovak market for imported vehicles focuses mainly on newer cars, with the largest shares being vehicles up to 10 years old.

In recent years, the issue of emissions associated with diesel engines (diesel engines) has become a key topic in the field of the automotive industry and the environment. Diesel engines are known for their efficiency and higher fuel efficiency compared to petrol engines (petrol engines). However, they also produce higher amounts of harmful substances, such as nitrogen oxides (NO_X) and fine particles, which have a negative impact on air quality and human health [38,39].

Compared to gasoline engines, which produce higher amounts of carbon dioxide (CO₂), diesel engines contribute more to air pollution with harmful substances that are directly responsible for smog and respiratory diseases. This fact has led to the tightening of emission standards for diesel engines in many countries, especially in Europe. Foreign policies are gradually restricting the operation of older diesel vehicles in cities by banning them from entering low-emission zones, leading to a decrease in demand for these vehicles in the West. As a result, these vehicles are often sold to markets such as Slovakia, where they are still popular and affordable [40,41].

The following graph shows the number of imported vehicles in individual years. The analysis is focused exclusively on diesel vehicles. The chart is directly linked to Figure 3, which analyzes the total number of vehicles. The data in the graph represent the part corresponding to vehicles with a diesel engine.

The graph (Figure 4) shows the analysis of the age of imported diesel vehicles in the monitored period from 2018 to 2021. The horizontal axis shows individual years, while the vertical axis shows the percentage representation of different age categories of vehicles. The category “under 5 years” has the largest representation among imported diesel vehicles, reaching the highest values in all the monitored years. In 2018, this share was 6.74%; in 2019, it rose to a maximum of 8.78%, which is the highest value in the entire monitored period. In 2020, the share decreased slightly to 7.74%, and in 2021, it reached 7.66%. This trend indicates continued interest in newer diesel vehicles. Vehicles aged “from 5 to 10 years” constitute the second most numerous category. In 2018, they reached a value of 6.65%; in 2019, they fell to 4.56%; in 2020, they rose slightly to 3.70%; and in 2021, they rose again to 3.97%. This category shows a decrease and a subsequent increase in the monitored period, while the values remain stable in the range of 3.5% to 6.6%. The category of vehicles “from 10 to 15 years” shows similar values throughout the entire period. In 2018, it was 5.61%; in 2019, it reached 6.18%, which is the maximum for this category. In 2020, it decreased slightly to 3.50%, and in 2021, it increased to 3.50%. Other age categories, such as “from 15 to 20 years old”, “from 20 to 25 years old”, and “over 25 years old” have relatively low shares and are below 2% in all the years. The category “from 20 to 25 years old” reaches the lowest values, with a share of less than 0.5% in all the monitored years. Overall, the graph shows that the import of diesel vehicles to Slovakia is focused primarily on younger vehicles, with the largest number of vehicles under 5 years old. Older vehicles make up a minor part of imports, which may be a consequence of legislative measures or a change in consumer preferences towards newer and more environmentally friendly vehicles.

Looking at the comparison of the import of diesel vehicles with the total import of vehicles in the monitored period (Figure 5), we can see that diesel vehicles represent the majority of the total import of vehicles. In 2018, diesel vehicles under 5 years old accounted for 6.74% of the total number of imported vehicles, while the total share of all vehicles under 5 years old was 10.02%. This indicates that diesel vehicles under 5 years of age make up a substantial part of the total imports in this age category. Diesel vehicles from 5 to 10 years old accounted for 6.65%, which was a similarly significant share compared to the overall figures. In 2019, the share of diesel vehicles under 5 years old rose to 8.78%, while the total share of all vehicles under 5 years old was 10.41%. This again shows that diesel vehicles are the dominant vehicle type in this age category. A similar trend can be observed for vehicles from 5 to 10 years old, where diesel vehicles represented 4.65% of the total imports. The year 2020 confirms this trend when diesel vehicles under 5 years of age accounted for 7.74% of total vehicle imports, which again is a significant part of the total share of all vehicles under 5 years of age, which was 9.52%. Even this year, we see that diesel vehicles are also significantly represented in the category of vehicles from 5 to 10 years old, representing 3.70% of the total imports. In 2021, diesel vehicles under 5 years old accounted for 7.66% of total imports, while the total share of all vehicles under 5 years old was 10.00%. Diesel vehicles from 5 to 10 years old reached a share of 3.97%, which again confirms that diesel vehicles make up the majority of the total imports in this category. The comparison shows that diesel vehicles represent the majority of the total import of vehicles in the monitored period, especially in the categories of newer vehicles (up to 5 years) and middle-aged vehicles (5 to 10 years). This trend indicates that despite the gradual change in consumer preferences and the pressure to reduce emissions, diesel vehicles still maintain their dominant position in the imported vehicle market.

Due to the current trends in Western European markets, where diesel vehicles are gradually regulated and restricted, they are becoming less attractive to consumers. Many cities and countries are imposing stricter emission standards and restrictions on vehicles with older diesel engines, leading to a decrease in their value and increased sales of these vehicles in secondary markets. This development has a direct impact on Slovakia, where diesel vehicles are imported in large quantities. The Slovak vehicle trading market often takes advantage of this situation to purchase these vehicles at relatively lower prices. As the data show, diesel vehicles make up the majority of the total imports of vehicles to Slovakia. For example, in 2018, diesel vehicles under 5 years of age accounted for 6.74% of the total number of imported vehicles, while in 2019, this share increased to 8.78%. Even in the following years, 2020 and 2021, diesel vehicles continued to represent a significant share of total imports, with shares of 7.74% and 7.66% for vehicles under 5 years old. This trend indicates that Slovak consumers are actively buying diesel vehicles, which are being phased out in the West due to stricter emission regulations. For Slovaks, where emission regulations are less strict, these vehicles represent an attractive option at affordable prices, which contributes to their dominance in the market for imported vehicles. Considering the above trend results, it is clear that the import of older used vehicles contributes to the increase in the overall level of air pollution in Slovakia. This should create pressure for continued improvements in regulatory measures, such as stricter emission inspections, the introduction of low-emission zones in cities, and incentives to purchase greener vehicles such as electric and hybrid cars.

In response to these challenges, Slovakia should actively strive to improve the regulation of the import of older vehicles and introduce stricter measures limiting their negative impact on air quality. These activities can include, for example, increasing registration fees for vehicles with high emissions, stricter (more frequent) emission inspections, or providing subsidies for the purchase of new, environmentally friendly cars.

4. Evaluation of Emission Inspection Data—Methodology

The research methodology is based on a detailed examination of data from the process of emission inspections carried out on passenger vehicles in the Slovak Republic. The processing of emission inspection data is key to obtaining an overview of the emission status of vehicles that are operated on the roads every day [26]. For the purpose of research processing, data on the course of emission inspections were obtained from the authorized technical service for carrying out emission inspections in the Slovak Republic. The data are generated nationwide from the results of emission inspections. The results contain records of the number of inspected vehicles, types of fuels used, age categories of vehicles, and the severity of faults that were identified during the inspection. All the data are recorded in a uniform form and categorized according to uniform parameters, such as fuel (petrol, diesel, alternative fuels), vehicle status (temporarily capable, incapable, capable), and fault codes according to a specified code list. Recording took place at the level of emission inspection sites, where data were directly entered into a nationwide electronic system with an emphasis on accuracy and consistency [26].

Given the scope of the data, the most important data from the research perspective were selected and subsequently evaluated. The basic variables included data such as the following:

• Type of fuel: gasoline, gasoline–gas fuel, diesel, and alternative fuels.
• Vehicle age: for research purposes, vehicles were divided into individual categories by age: up to 5 years, 5–10 years, 10–15 years, 15–20 years, 20–25 years, and over 25 years.
• Emission inspection result: the result is determined according to specific procedures and a set of exhaust gas measurements according to the applicable legislation [26]. Vehicles can be categorized as temporarily eligible (the operator has 2 months to eliminate the defect), ineligible, and eligible.
• Types of faults found during the emission inspection: faults are categorized according to severity and cause based on the fault code, which divides faults into categories A, B, and C:
  • Category A includes malfunctions that do not affect the functionality of the vehicle and do not have a direct impact on emissions. These faults are considered minor, and the vehicle can still be considered roadworthy.
  • Category B includes moderate faults that may have some impact on vehicle functionality and potential emissions. A vehicle with a category B fault may be temporarily eligible but requires repair within a specified time.
  • Category C includes the most serious faults that directly affect the vehicle’s emissions and its overall capability. Vehicles with a Category C defect are considered unfit and cannot continue to operate until the defects are corrected.

The research primarily focuses on evaluating individual types of failures depending on the age of the vehicle and the fuel used. For each category, the percentage of occurrence of specific failures was determined, while the results were compared and evaluated between individual fuel types and vehicle age groups. The processed data allow the identification of trends, such as the increase in unfulfilled emission criteria for older vehicles and differences in the success of emission controls depending on the type of fuel. The quality and accuracy of the obtained data were ensured by the regular checking of records and the validation of input data at the emission stations. Data collection took place under controlled conditions, thereby minimizing the possibility of error. During the evaluation, control mechanisms were used to identify and exclude non-standard records that could distort the results. Based on the results of the study, it is possible to effectively set regulatory and control measures to improve the environmental parameters of vehicles.

In the following section, you can follow a detailed analysis of the results of the emission controls that were carried out in the Slovak Republic in 2023. This is an evaluation of emission inspections that were carried out by all workplaces. The research focuses exclusively on emission inspections of passenger vehicles intended for the transport of people. The number of vehicles inspected at emission inspection sites was 1,032,641. The graphic representation can be seen in the following image.

5. Research Results

In the following section, it is possible to follow a detailed analysis of the results of emission inspections carried out in the territory of the Slovak Republic in 2023. These are emission inspections that were carried out by all workplaces. Only the results of M1 category vehicles will be analyzed. The number of regular inspections carried out at emission inspection workplaces was 1,032,641. The graphical expression of the results is supplemented by tabular processing, which contains the source data.

Figure 6, supplemented by the source table (

Table 2. Results of emission inspections—source data.

	Temporarily Eligible	Ineligible	Eligible	Total
Number of performed inspections—absolute statement	18,937	39,954	973,750	1,032,641.00
Number of performed inspections—relative expression	1.83%	3.87%	94.30%

), shows the results of emission inspections in the Slovak Republic. The largest share of vehicles, namely 94.3%, was classified as fit for further operation, which represents a clear peak in the graph. This indicates that the majority of vehicles that passed the emission inspection met the established emission limits. Conversely, the share of unfit vehicles that were assessed as unfit for further operation is only 3.87% (in absolute terms, their number is 39,954 vehicles), which is the lowest value in the graph. The category of temporarily fit vehicles, which represents vehicles that can be temporarily operated if the identified deficiencies are eliminated, constitutes 1.83% (18,937 vehicles) of all the inspected vehicles. Overall, the graph indicates a high level of compliance with emission standards during emission control performance, with only a small proportion of vehicles failing emission controls.

The graph (Figure 7) shows the results of emission inspections in Slovakia, broken down by vehicle age. The largest share of vehicles that successfully passed the emissions test is in the 5–10-year and 15–20-year categories, where the eligible vehicles represent 26.67% and 26.31%, respectively. These values are the highest among all the age categories, indicating that vehicles in these age groups are most compliant with emission standards. Conversely, the lowest share of eligible vehicles is in the over-25 category, where only 5.04% were eligible, which is also the lowest value in the graph. This group also shows the highest share of unqualified vehicles (1.42%). The categories of temporarily eligible vehicles are represented by only very low values in all age groups, with their share ranging from 0.09% to 0.99%, with the highest value in the over-25 group. Unfit vehicles are most prevalent in the older categories, specifically in the over-25 age group, where they reach 1.42%, while in younger age groups, this proportion is much lower. Overall, the graph shows that with the age of the vehicle, the probability that the vehicle will be assessed as ineligible during the emissions test increases. For this reason, the influence of the vehicle’s age on the emission inspection result can be observed in the following figure. The graph presents data on the age of vehicles that were assessed as ineligible at emission inspection workplaces in a given year. Their total number was 39,954 vehicles.

The graph (Figure 8) shows an overview of vehicles assessed as ineligible, combining two main axes: the left axis shows the absolute values of ineligible vehicles. In contrast, the right axis shows the percentage of the total number of vehicles assessed as ineligible. The course of the graph shows an increase in the number of ineligible vehicles with age, reaching the maximum in the category of vehicles aged between 15 and 20 years, where the percentage expression reaches 36.70%. Subsequently, there is a decrease, while the lowest values are recorded for the youngest vehicles (up to 5 years) and the oldest (over 25 years). This trend indicates that the highest proportion of ineligible vehicles is found in the middle age of the vehicle.

When compared to the previous graph, which shows the results of emission inspections according to the age of the vehicle, it is possible to identify a similar trend, where older vehicles show a higher proportion of ineligibles. The results show that as the vehicle ages, the probability of inoperability increases, which may be a result of deteriorating technical conditions and higher wear of components, which can affect emissions production. This trend is probably due not only to the aging of vehicles but also to a possible lack of regular maintenance, which increases the risk that a vehicle rated as unfit for emission inspection will be considered unfit for operation.

5.1. Analysis of Data from Emission Inspections by Type of Fuel

Analyzing the vehicles assessed as ineligible at the emission inspection and their distribution by fuel type is crucial in terms of environmental impacts. Diesel engines, known for higher emissions of nitrogen oxides (NO_X) and particulate matter, pose a greater risk to air quality compared to petrol engines, which produce more carbon dioxide (CO₂). Such an analysis makes it possible to better understand and solve the sources of pollution and take targeted measures to reduce emissions of harmful substances, thereby contributing to the protection of public health and the environment.

In the following section, the results of the emission inspections can be monitored based on the division of vehicles into groups according to the fuel they burn during operation. The graphic course presents the percentage composition of the vehicles that participated in the emission inspection. Absolute values are given in the table.

The graphic process (Figure 9) shows the composition of the vehicles that came for the emission inspection performance, divided by fuel type. The graph is supplemented by

Table 3. Composition of vehicles in emission inspections by type of fuel—source data.

	Temporarily Eligible	Ineligible	Eligible	Total
Gasoline	10,354	23,651	467,215	501,220
Gasoline—gaseous fuel	1147	2766	20,626	24,539
Diesel	7414	13,520	485,261	506,195
Alternative fuel—gaseous fuel	22	19	648	689
Total	18,937	39,956	973,750	103,2643

, where it is possible to monitor the data on the total number of inspected vehicles—an absolute expression. It is clear from the graph that the most significant representation is vehicles with a diesel engine (49.019%), closely followed by vehicles with a gasoline engine (48.538%). Vehicles with a gasoline engine also use gaseous fuel, representing 2.376%, and vehicles with an alternative fuel use only 0.067%. The results of the research reflect the dominance of diesel and gasoline engines in Slovakia, while the combination of gasoline and gas fuel forms a smaller but still significant part. A comparison with the previous chart on emissions test results by vehicle age suggests that diesel vehicles, which make up almost half of all vehicles tested, may be the main source of unsatisfactory emission results, especially given their higher proportion of NO_X and particulate emissions. This fact may explain the need for stricter emission inspections for diesel engines in order to minimize their negative impact on the environment. This image alone shows the need for stricter emission controls for diesel engines in order to minimize their negative impact on the environment.

In the following graph, it is possible to observe the results of checks for vehicles with individual types of drive. For a better presentation of the results, the graph presents the results in percentage terms. Each drive category represents a separate whole 100%. From the data, it is possible to follow in detail the evaluation of the emission inspection by the technician for each type of vehicle separately.

The graph (Figure 10) shows the results of vehicle emission inspections in Slovakia, divided by the type of fuel used. In the case of gasoline engines, 93.216% of the vehicles are eligible, while the temporarily eligible makeup 2.066% and the ineligible 4.719%. Vehicles with a gasoline engine and gas fuel show a lower proportion of eligible vehicles (84.054%) and a higher proportion of ineligible vehicles (11.272%). Diesel engines are capable at the level of 95.864%, while ineligible ones make up 2.671%. Alternative fuel vehicles with gaseous fuel achieve the highest eligibility (94.049%) with the lowest proportion of ineligible vehicles (2.758%). When comparing the monitored data, it can be observed that vehicles burning alternative fuels achieve almost the highest efficiency in emission inspections, which may be due to the purity of burning alternative fuels. Diesel engines, despite their higher susceptibility to producing harmful emissions, show a high degree of competence, which indicates the effectiveness of modern filters and emission reduction technologies. Conversely, gasoline engines, especially those combined with gaseous fuel, have a higher proportion of ineligible results, which may be related to the diversity of the quality and maintenance of these systems.

The following graph shows the data specifying the results of the emission inspections of vehicles that burn the most widespread type of fuel. In the Slovak Republic, the majority of vehicles are still powered by fossil fuels—gasoline and diesel.

The graph (Figure 11) shows the results of emission inspections in Slovakia according to the fuel used. In the case of gasoline-burning vehicles, the percentage of eligible vehicles that were rated as eligible at the emissions test is 93.216%. The lowest values are recorded for the group of vehicles evaluated as temporarily eligible, with a value of 2.066%, and ineligible, with a value of 4.719%. Diesel vehicles show a slightly higher share of eligible vehicles than gasoline vehicles, with a value of 95.864%. The rate for temporarily eligible vehicles is lower (1.465%) than for gasoline vehicles. The percentage of vehicles rated as ineligible is 2.671%, which is lower than for gasoline vehicles. The overall results indicate that diesel vehicles are more likely to pass the emissions test than gasoline vehicles, but their differences are insignificant.

The following graph shows an overview of the vehicles rated as ineligible due to age and type of fuel. In this comparison, the vehicles assessed as ineligible are included in the monitoring. For a better representation, the graphic course is shown in percentage terms. In absolute terms, it is possible to track the total number of vehicles assessed as unsuitable for the respective fuel type. The data can be found in Table 3.

The graph (Figure 12) shows an overview of the vehicles that were assessed as ineligible based on age and type of fuel (gasoline or diesel). For petrol vehicles, disqualification starts at a very low level for the youngest vehicles under 5 years of age, at 0.05%. This share of ineligible vehicles increases with age, reaching its maximum in the 15–20-year category, where 33.07% of the vehicles are ineligible. After that, this share gradually decreases to 16.79% for vehicles older than 25 years. Diesel vehicles show a similar trend, with very low ineligibility for the youngest vehicles under 5 years old (0.09%). Ineligibility increases with increasing age, with a maximum in the 15–20 age category, reaching 42.75%, which is higher than gasoline vehicles. In the category of vehicles over 25 years old, the share of ineligible diesel vehicles drops significantly to 4.40%, which again is lower than gasoline vehicles.

The overall results indicate that older diesel vehicles are more likely to be rated as ineligible compared to petrol vehicles, with this difference being most pronounced in the middle age of the vehicles (10 to 20 years).

5.2. The Most Frequent Malfunctions Recorded During the Performance of Emission Inspection

Emission inspections in the Slovak Republic are an important part of regular vehicle maintenance, ensuring that vehicles meet environmental standards regarding emissions of harmful substances into the air. The aim of these checks is to minimize the negative impact of motor vehicles on the environment and to ensure that vehicles on the road meet the prescribed emission limits. During the emissions test, various aspects of the motor vehicle are examined, including its ability to properly process and filter harmful substances.

Emission inspection failure recording is a systematic process that involves identifying and classifying defects based on the severity and type of problem. Faults are divided into three main categories: A, B, and C. For accurate and uniform recording of errors detected during emission inspections, a “code book” of faults is used, which is a standardized system for marking and classifying detected problems. This codebook ensures that technical personnel evaluate detected faults in the same way—by code during emission inspections. Based on access to the national database of results and a uniform evaluation of emission checks, it is possible to monitor and analyze vehicle emission status faults in detail. The code book can be found at the end of the research in

Table A1. Code list for the unification of faults detected during the emission inspection.

Item Description	Breakdown of Item Description	Fault Description	Classification	Fault Code—Internal	Fault Code—Harmonized
GASOLINE	Measurement (410)	CO or HC at idle speed and CO or λ at increased speeds outside the specified range	C	411	8.2.1.2.SK.1
		CO with fuel gas outside the specified range	C	412	8.2.1.2.SK.2
		idling and control speeds outside the specified range	B	413	8.2.1.2.SK.3
		current, voltage, λobd, ripple voltage, out of specified range	B	414	8.2.1.2.SK.7
		CO2, O2, and λ at idle outside the specified range	B	415	8.2.1.2.SK.6
		the increased revolutions cannot be kept within the specified range	B	416	8.2.1.2.SK.8
		CO2, O2, and λ at idling outside the specified range and const. engine have an effect on their size	A	417	8.2.1.2.SK.10
DIESEL	Measurement (510)	smokiness higher than the specified range	C	511	8.2.2.2.SK.1
		maximum revolutions outside the specified range	B	512	8.2.2.2.SK.2
		idling speed higher than the specified range	B	513	8.2.2.2.SK.3
		dispersion of absorption coefficients outside the specified range	B	514	8.2.2.2.SK.4
		disproportionate increase in smoke between accelerations	C	515	8.2.2.2.SK.5
		the dispersion of times evaluated by acceleration is higher than the specified value	A	516	8.2.2.2.SK.6
ECU—OBD	General (610)	it is not possible to establish communication in specific cases	A	611	8.2.SK.4
		communication cannot be established	B	612	8.2.SK.5
		readiness code unable to load test rating status	B	613	8.2.SK.6
		the MIL status cannot be read or there is a mismatch between the read and actual MIL status	B	614	8.2.SK.7
		MIL status indicates a malfunction (flashes, lights up)	C	615	8.2.SK.8
	Fault memory (620)	Unable to read OBD memory	B	621	8.2.SK.9
		fault P0..., P2—vehicle with petrol engine	B	622	8.2.1.2.d
		fault P1..., P3...,	A	623	8.2.SK.10
		discrepancy between the VIN on the vehicle and the recorded one (detection of OBD identification data)	A	624	8.2.SK.11
		fault P0..., P2—vehicle with a diesel engine	B	625	8.2.1.2.d

—Appendix A.

5.2.1. Temporary Ineligible Vehicles

Vehicles that have been found to have a serious fault during the emission inspection are temporarily ineligible for road traffic. If the emission inspection reveals a serious fault, the operator is obliged to submit the vehicle to a repeated emission inspection within 60 calendar days. If the same serious fault is detected again by repeated emission inspection, the vehicle is unfit for operation in road traffic. If the vehicle operator does not submit the vehicle to repeated emission inspection within the established period, the vehicle is unfit for operation in road traffic.

The following graph shows the most frequently occurring fault in vehicles, as a result of which these vehicles were assessed as temporarily unfit for emission inspection. The results in the graph are presented in percentage terms. The total number of vehicles assessed as temporarily ineligible is 18,937.

The graphic representation in Figure 13 shows the most frequent failures recorded during emission inspections that led to the vehicle being temporarily ineligible. The largest share, up to 40.57%, belongs to the “Fault memory” category, which indicates that the most common problem is errors recorded in the fault memory in the engine control unit. The second most common failure, “Measurement”, reaches 33.39% and points to a high incidence of problems associated with emission measurement—for example, the idling speed is outside the specified range. Defects detected during a visual inspection, categorized as “Visual inspection”, represent 24.35% of cases—for example, leakage of operating fluids. The least frequent is the general faults marked as “General”, which make up only 1.70% of the recorded faults—for example, the use of non-approved components in the vehicle. It is clear from the results that the biggest problems are caused by faults in electronic systems and diagnostics, while visual disturbances and general faults are less significant. These data point to the need to improve systems for detecting and correcting diagnostic and emission problems in order to minimize the temporary ineligibility of vehicles.

5.2.2. Ineligible Vehicles

Vehicles in which dangerous defects were detected during the emission inspection are ineligible for operation in road traffic, and it is necessary to prohibit the use of the vehicles in road traffic. The following graph shows the most frequently occurring fault in vehicles, as a result of which these vehicles were assessed as ineligible during the emission inspection. The results in the graph are presented in percentage terms. The total number of vehicles assessed as temporarily ineligible is 39,954.

The pie chart (Figure 14) shows the distribution of the most frequent fault recorded during emission checks that resulted in vehicles being classified as unfit for operation. Each segment of the graph represents the share of one particular fault, with the faults marked by their code and severity category (category B or C). The most prominent part of the graph, which occupies almost half of the total volume (54.93%), represents the fault “measurement,411, C”—the direct measurement of emissions for vehicles with a gasoline engine, indicating that this fault is dominant and significantly prevails over the others, which indicates that this error is dominant and significantly prevails over the others—for example, the production of emissions is higher than the value set by the manufacturer (the vehicle excessively pollutes the air). The second most common fault, “measurement,511, C”, is the direct measurement of emissions for vehicles with a diesel engine, which occupies 29.65%, which is also a significant share. However, it is roughly half that of gasoline vehicles—for example, the production of emissions is higher than the value set by the manufacturer (the vehicle excessively pollutes the air). The remaining faults have a significantly smaller representation. In contrast, the smallest share is the fault “visual,264, B” with a value of 0.81%, which results in the fact that it is not possible to perform a visual inspection or measurement of the vehicle. Overall, the graphical results clearly show that errors related to direct emission measurement are the most common reason why vehicles are assessed as unfit during emissions testing. Due to the fact that in the Slovak Republic, there is the largest representation of vehicles with a conventional combustion engine for fossil fuels (gasoline, diesel), only this group of vehicles is analyzed in the following section. In the following graph, it is possible to monitor the occurrence of faults that result in increased production of emissions. This group of vehicles represents vehicles that were assessed as ineligible during the emission inspection. Their total number is 35,715. Compared to the total number of ineligible vehicles, there was a decrease of 4239 vehicles. This difference is natural and represents a group of vehicles that were assessed as ineligible, but it was not caused by the direct measurement of emissions. The graphic expression is supplemented by the source in the table, which makes it possible to follow the absolute expression of the presented data.

The graph (Figure 15 and

Table 4. Absolute expression—malfunctions detected during emission inspection—increased production of emissions.

	Up to 5 Years	From 5 to 10 Years	From 10 to 15 Years	From 15 to 20 Years	From 20 to 25 Years	Over 25 Years	Total
Diesel	19	1382	4731	7151	2785	570	16,638
Gasoline	9	1058	2239	6341	6307	3123	19,077

) shows the faults detected during the emission inspection that led to increased emission production. In contrast, the faults are divided according to the vehicles’ age and the fuel type (diesel or petrol). When evaluating the graph, it can be observed that the highest proportion of faults leading to increased emission production was recorded in the category of vehicles aged 15 to 20 years, where diesel vehicles show a value of 20.02% and petrol vehicles 17.75%. This indicates that older vehicles, especially diesel ones, are more likely to produce increased emissions. Conversely, the lowest share of breakdowns is recorded for vehicles up to 5 years old, where it represents only 0.05% for diesel vehicles and even only 0.03% for gasoline vehicles. This result is expected, because newer vehicles have modern technologies that better control and reduce the preparation of the mixture and thus the generation of emissions. Overall, it can be concluded that as the age of the vehicle increases, so does the probability of malfunctions leading to increased production of emissions, while diesel vehicles tend to show a higher proportion of these malfunctions compared to gasoline vehicles, especially in the age categories from 10 to 20 years.

6. Discussion

In Slovakia, there is a decrease in the number of passengers on public transport, while individual passenger transport is recording a significant increase. This effect indicates a change in the preferences of the population in favor of their own mobility [42]. This fact is worrying in terms of the impact on air quality since vehicles with combustion engines are known to produce a higher amount of harmful emissions, especially nitrogen oxides and particulate matter. From the point of view of the import of used vehicles, it can be said that vehicles up to 10 years old are imported to the territory of the Slovak Republic. A significant part of the imported vehicles are diesel-powered vehicles. A similar situation is also pointed out in the study by Kendra et al., who showed in their research that the composition of the vehicle fleet significantly affects the total amount of emissions produced [43]. On the other hand, reducing the age of the vehicle fleet is one of the opportunities that countries have to limit greenhouse gas emissions. Unlike other European countries, preferential treatment for more environmentally friendly motor vehicles does not work in Slovakia. As a result, vehicles whose age exceeds the moral lifespan of the vehicle are imported to Slovakia, and Slovakia is turning into a junkyard in Europe. Statistics from the Association of the Automotive Industry indicate that in 2021, imports of used passenger cars to Slovakia, in contrast to purchases of new vehicles, have been steadily increasing. While new vehicle registrations decreased by 0.79 percent or 605 cars to 75,700 year on year, used car imports increased by 3846 cars (6.3 percent) to 61,032 year on year. Although Slovakia is a global automotive powerhouse, boasting the highest per capita car production in the world year after year, most of these cars are destined for wealthier markets [44].

A comprehensive review of the results of emission controls in the Slovak Republic provides a unique insight into the composition of vehicles and the effectiveness of current measures. The research results show that a total of 94.3% of vehicles are classified as roadworthy, while only 3.87% were assessed as unroadworthy, with a significant proportion of diesel vehicles among them. Studies from European countries with stricter legislation confirm a similar trend—older diesel engines are the main source of higher emissions of nitrogen oxides (NO_X) and particulate matter, which have a significant impact on air pollution and public health. For example, research in the UK and France found that cities that introduced strict controls and low-emission zones were able to reduce NO_X concentrations by more than 20% [45,46]. In such cases, it is also necessary to point out a more stringent form of emission inspection. In these countries, the AU Guide 6 is in force, which focuses on counting particulate matter contained in exhaust gases. Such measurements are required for vehicles with Euro 6/VI diesel engines because they have a particulate filter. In the future, it will be possible to determine whether and how effectively the respective particulate filter is working. Conventional turbidity measurement for Euro 4 and Euro 5 vehicles will no longer be sufficient for Euro 6 vehicles from 1 July 2023 at the latest [47,48]. In Slovakia, no change in the emission inspection process is currently being considered. Nevertheless, the results show that older vehicles, especially those between 10 and 20 years old, have the highest rate of non-compliance with emission standards, which is especially true for diesel vehicles with a share of up to 42.75% of non-compliant vehicles in the 15- to 20-year category. Compared to petrol vehicles, which have a share of 33.07% in the same category, diesel engines are particularly problematic, which may be due to their tendency to produce higher levels of NO_X and particulates, which directly contribute to smog and affect air quality. In the next part, the research focused on the breakdown of vehicles by age and fuel type in the context of emissions failures. The results showed that older vehicles, especially those between 15 and 20 years old, have the highest probability of being assessed as unfit. Diesel vehicles in this age category showed values of up to 42.75%, while petrol vehicles reached 33.07%. These data point to the fact that older vehicles are more prone to failures that lead to unsatisfactory emissions values. As for the emission testing itself, as part of the emission inspection, the vehicle is subjected to a check of the engine control unit’s fault memory. Several studies indicate an increase in emission production due to damage to a component in the engine. The conclusions of the research represent important findings from the field of engine management and fuel mixture preparation. At the same time, they show the impact of individual electronic components in the combustion engine on engine performance and exhaust gas emission control. By their proper functioning, we can prevent excessive production of exhaust emissions into the air. The proper functioning of the components and the entire vehicle also contributes to overall road safety. In the event of such a failure, the driver may not be aware of the incorrect function, and the vehicle excessively pollutes the air during its operation [49,50]. The conclusions of the research represent important findings from the field of engine management and fuel mixture preparation. At the same time, they show the impact of individual electronic components in the combustion engine on exhaust gas emission control. Their proper functioning can prevent excessive production of exhaust emissions into the air. The proper functioning of components and the entire vehicle also contributes to overall road safety.

The results of the study can be used in the future as a basis for vehicle inspections and to identify critical areas where emission performance could be improved. The evidence of a high proportion of non-compliant older diesel vehicles may prompt the introduction of regulations that support fleet renewal through incentives for the purchase of cleaner vehicles [36,51,52]. Targeted measures, such as increasing registration fees for older, high-emission vehicles or subsidies for the purchase of electric and hybrid vehicles, may contribute to reducing air pollution and achieving the country’s environmental goals in the future [53]. The overall results of the study clearly demonstrate that vehicle ageing is associated with an increased likelihood of deterioration in emission performance, with diesel engines showing a higher tendency to break down and increase emission production. These findings point to the need for stricter control measures for older vehicles and support for innovation in emission analysis technologies in order to improve air quality and achieve environmental goals.

7. Conclusions

The study is aimed at evaluating the results of emission inspections in the Slovak Republic with the aim of identifying the factors influencing their formation and also the steps based on which it is possible to meet the objectives of the European Union and permanently reduce emissions from the transport sector. Data on the import of used vehicles show that the largest group among imported vehicles in the monitored period is made up of vehicles in the age category “up to 5 years”. The second most numerous category is vehicles aged “from 5 to 10 years”. A comparison of diesel vehicle imports with total imports in the monitored period shows that diesel vehicles represent the majority of total imports, especially in the categories of newer vehicles (up to 5 years) and middle-aged vehicles (5 to 10 years). The results of emission inspections clearly indicate that older vehicles can pose a significant environmental risk unless they are equipped with modern technologies for reducing emissions. Furthermore, an analysis of faults detected during emissions checks showed that the most frequently identified problems were failures of measurement systems, which play a key role in determining whether a vehicle meets emissions standards. In particular, the quality of these systems may be insufficient in vehicles imported from other countries, leading to an increased rate of failures and subsequent failure of emission inspections. The conclusions of the study point to the need for more rigorous control mechanisms when importing older vehicles into the Slovak Republic. Their aim should be to ensure that the emission standards of imported vehicles are met. Based on the processed data, it is possible to gradually introduce stricter control mechanisms for older vehicles, support the renewal of the vehicle fleet, and implement modern technologies to reduce emissions. Consistent emission inspections are essential for the protection of public health. In the future, it is also necessary to monitor regional differences as well as differences between individual countries. The application of the knowledge gained could lead to a deeper understanding of the issue of vehicle emissions and to more effective measures to improve air quality.