Reasons for failing the vehicle inspection due to emissions and how to avoid them

You arrive at the vehicle inspection station, you've looked the car over from top to bottom, and you're relatively calm. You've checked Lights, license plate, tires, brakes, and even those strange noises They appear when it rains or you hit a pothole. You keep up with the maintenance, so you decide to go yourself, without stopping at the garage first. After a long wait, it's finally your turn.

You enter the pit, the technician checks the documentation, connects the probe to the exhaust, asks you to accelerate fully and, suddenly, comes the cold shower: Failed MOT test due to emissionsAs they continue with the rest of the checks, you start hearing concepts that sound like another language: opacity, Lambda sensor, high CO, EGR, DPF… When you leave, you have a report full of defects and a very clear question: What has happened with the emissions and why are they such a frequent cause of rejection?

Possible results in the MOT test and what it means to fail due to emissions

In a technical inspection there are only three possible outcomes: favorable, unfavorable, or negativeThis is not a minor detail, because it determines how you can drive and how much time you have to fix the car.

When the result is If favorable, the vehicle can continue driving normally. until the next mandatory inspection. You have passed all sections (brakes, steering, suspension, lighting and, of course, emissions) within legal limits.

Things change when the ruling is unfavorableIn this case, serious defects have been detected, but the car is not considered dangerous enough to warrant immediate impoundment. What does this mean? That you can only drive it. to go from the vehicle inspection center to the garage and from the garage back to the vehicle inspection center for the second inspection. Do not use the car normally in the meantime.

The worst-case scenario is the negative MOTHere we are talking about extremely serious failures that pose an immediate risk to road safety or the environment. The vehicle is immobilized and It has to be towed out. directly to the repair shop. Until it is repaired and passes a new inspection, it cannot be driven on public roads.

Among all the possible serious defects, the Polluting emissions have become one of the most common reasons for rejectionThis applies to both diesel and gasoline cars, especially older vehicles or those with poor maintenance. Recent studies have identified it as the leading cause of failing grades in many passenger cars.

Facts: why emissions are such a frequent reason for failing the MOT test

Far from being a rare fault, gas rejections are becoming increasingly common. According to various reports from official bodies and industry associations, around 2 out of 10 vehicles fail the MOT test on the first attemptAnd, within them, the emissions section is gradually climbing the ranks to become one of the leaders in serious defects.

For example, in certain autonomous communities, records indicate that nearly one 17,5% of vehicles fail inspection due to serious defectsMany of these are directly linked to emissions control. Other studies mention that emissions testing is responsible for approximately 8,8% of inspections were unfavorable in some periods, a percentage that has continued to grow over the years.

In more recent reports from entities such as AECA-ITVThe picture is even clearer: emissions have come to represent in passenger cars around 16,95% of rejections in certain years, following an upward trend. This is due to two main factors: the tightening of legal limits in the stations and the progressive aging of the vehicle fleet, with increasingly older and worn-out cars.

The most recent data published by the association highlights that, in 2023, the Pollutant emissions already account for around 21,5% of serious defects in vehicles in general, rising to as high as 25,7% in the specific case of passenger cars. In other words: One in four cars that fails the test does so because they do not meet emissions limits..

All of this is also linked to another worrying reality: The average age of the vehicle fleet now exceeds 15 years. In Spain, the older the vehicle, the greater the likelihood that vital systems such as the catalytic converter, particulate filter, EGR valve, or injectors will not function properly and will cause pollutant emissions levels to spike during testing.

Other common causes of failing the MOT test (besides emissions)

Although the hot topic lately is the Reasons for rejection at the ITV (vehicle inspection) due to emissionsIt should not be forgotten that most failures are concentrated in a few key sections related to safety. Data from AECA-ITV and other organizations coincide: lighting, tires, suspension and brakes These, along with emissions, are the main protagonists of the unfavorable reports.

One of the blocks with the most defects is the one in lighting and signageSeemingly insignificant faults, such as a burnt-out bulb, a poorly adjusted headlight, or a license plate light that doesn't illuminate, can ruin the entire inspection. Statistics show percentages close to [percentage missing]. 25% of serious defects in this section, which makes it one of the undisputed leaders in suspense.

Another common source of problems are the axles, wheels, tires and suspensionThe tread wear, cracks or deformations, model or size differences on the same axle, and the overall condition of shock absorbers and suspension components are all checked. Serious defects here typically involve around... 19-20% of the total, with many cases of tires below the legal limit of 1,6 mm or with very uneven wear.

The section of brakes It also usually appears at the top of the list. The inspection checks include braking effectiveness, wheel balance on the same axle, the operation of the handbrake, and the condition of brake discs, drums, pads, and fluid. A significant imbalance between the two front or rear wheels can be considered a serious defect and result in immediate failure of the vehicle inspection.

Finally, one cannot neglect the body and chassisAlthough it is not the area where the most defects are concentrated, a poorly attached bumper, a windshield with a significant crack in the driver's field of vision, or a seatbelt that does not fasten correctly are considered serious defects related to the passive safety of the vehicle.

How pollution is measured in the MOT test: gas tests and systems involved

The emissions test is not a mere formality; it is highly regulated, and each type of engine has its own procedure. In practice, the objective is to verify if The engine and all anti-pollution systems are functioning within the limits. that is set by the Euro standard corresponding to the vehicle.

In the gasoline cars The main measurements taken are the concentration of carbon monoxide (CO), unburned hydrocarbons (HC), and the Lambda index, which indicates whether the air-fuel mixture is correct. Measurements are taken at idle and, in many cases, at a high and stable engine speed. A high CO or an out-of-range Lambda value They are usually a sign of a failure in the fuel system, in the Lambda probe, in the catalytic converter or of a poor engine adjustment.

In the diesel vehicles, the star of the test is the opacity of the smokeThis measures the amount of visible particles emitted by the exhaust. The operator connects the opacimeter to the exhaust pipe and performs several hard accelerations at full power. If the average value exceeds the limit set for that specific engine, the result is unfavorable for emissions.

This is where elements such as the EGR (exhaust gas recirculation valve), turbo, anti-particulate filter (FAP/DPF) or the injectors. A clogged EGR valve, a DPF saturated with carbon deposits, or dirty injectors can drastically increase the amount of smoke emitted and turn a car that seemingly "runs well" into a clear failure for emissions during the MOT test.

In more modern vehicles, especially those from Euro 5 onwards, the vehicle inspection itself can also be performed. Read the car's control unit through the OBD port (on-board diagnostics). If active or recently cleared error codes affecting anti-pollution systems are detected, or if devices such as the DPF or EGR are found to have been fraudulently disabled, the inspection may be rejected even if the smoke or CO value is not noticeably high.

Furthermore, an engine malfunction light illuminated on the instrument panel The check engine light raises all the technicians' suspicions. It's very likely that the diagnostic system will be thoroughly checked, and if a fault in emissions-related components is confirmed, the outcome will be unfavorable.

Main technical reasons for rejection due to emissions in the MOT

When a vehicle fails emissions testing, the report usually details the specific reason. Behind those codes and descriptions lie a number of fairly common problems that are worth knowing. Many of them are due to Wear and tear due to age, poor maintenance, or carbon buildup due to a very smooth and urban driving style.

One of the usual suspects is the catalytic converter in poor condition In gasoline engines, if this component loses efficiency, it is unable to properly transform harmful gases (CO, HC, NOx) into less damaging compounds, and the test results spike. Defects such as high CO and altered Lambda readings often appear together.

In diesel engines, the star combination is the one formed by Clogging particulate filter, dirty or stuck EGR valve, and injectors with depositsWhen the DPF (Diesel Particulate Filter) doesn't regenerate properly, soot builds up and the car starts emitting more smoke than it should, especially under full load. If, in addition, the EGR (Exhaust Gas Recirculation) system isn't recirculating the exhaust gases correctly, combustion is impaired and the amount of polluting emissions increases.

Problems related to Lambda probes and oxygen sensors In gasoline engines, if the sensors are faulty or old, the ECU won't adjust the fuel mixture correctly, resulting in out-of-range emissions. These sensors don't always show clear signs of malfunction in everyday driving, but they become apparent during inspections.

In very old cars, with high mileage or an irregular maintenance history, several of these problems can occur simultaneously: Worn injectors, low compression, faulty ignition system (in gasoline engines), small exhaust leaks… The sum of all these factors ends up being reflected in an emissions measurement that exceeds the permitted level.

Meanwhile, the vehicles that have suffered deliberate manipulation of anti-pollution systems (Disposal of DPFs, EGR disconnection, aggressive remapping without emissions control, etc.) are making it increasingly difficult to cheat emissions tests. The combination of emissions testing and electronic diagnostics makes these tricks easier to detect, and when discovered, they result in an immediate failure.

How to get to the vehicle inspection station in the best conditions for the emissions test

While you can't perform miracles on a faulty engine, there are a number of simple guidelines that can help you go into inspection in much better condition. Part of the trick is that engine and exhaust reach their optimal operating temperature before the vehicle enters the pit.

Ideally, you should ask. appointment at the ITV (vehicle inspection station) and plan your arrival so that, just before the inspection, you can travel several kilometers by road or highway. During this journey, it is advisable to use slightly shorter gears than usual This allows the engine to operate at a medium-high RPM (without exceeding speed limits, of course). This increases the temperature of the exhaust gases and helps burn off some of the accumulated carbon deposits.

In a diesel vehicle, it's especially useful to drive for a while above the 3.000 rpm (always respecting the speed limit), especially if the car is used almost exclusively in the city. This combination of high revolutions and hot exhaust gases helps to... the particulate filter regenerates better and that the system expels accumulations of soot that would otherwise appear as black smoke during the vehicle inspection.

In general, it's also advisable that, while you wait your turn at the station, Don't leave the car off indefinitely.If the wait is long, you can start the engine a few minutes before entering the pit to allow it to warm up. If the engine and exhaust get too cold, especially in winter, the test is more likely to go worse than it could.

When the measurement fails by very little, in many stations it is possible repeat the gas test at the same time. Sometimes, simply performing a series of strong accelerations on the first attempt causes the catalyst or the DPF to reach its ideal temperature, and the second measurement yields clearly better values.

On the other hand, some drivers choose to use higher quality fuel or specific injection cleaning additives in the pre-MOT fuel tanks. Although they are not miracle solutions, in engines with moderate dirt they can help reduce deposits in injectors and combustion chambers, slightly improving the test results, especially if combined with journeys at good engine speeds and higher than usual temperatures.

What you should never do: cheating and tampering with anti-pollution systems

Fearing failure due to emissions, all sorts of "shortcuts" are circulating in disreputable workshops and internet forums to try to pass. cheating the vehicle inspectionFrom disconnecting sensors to prevent them from triggering errors to physically disabling the particulate filter or the EGR valve. They might get away with it for a while, but it's becoming increasingly risky and, moreover, irresponsible.

In Euro 5 and later vehicles, inspectors have the option to Check the control unit via the OBD portIf they detect that the anti-pollution systems are not working as they should, that there are recently cleared error codes, or that the operating values ​​do not match up, failure is practically automatic.

Any intervention that involves disable the catalytic converter, the particulate filter, or the EGR valveInstalling ECU reprogramming that significantly increases smoke and emissions without control is not only prohibited, but also exposes you to heavy penalties, insurance problems and, of course, immediate rejection at the MOT if the manipulation is detected.

Furthermore, these "tricks" almost always prove costly in the medium term. An engine pushed to its limits, or deprived of systems designed to control combustion and emissions, It tends to suffer more breakdowns and higher consumptionWhat seemed like a quick saving ends up being a greater expense in fuel, repairs and, eventually, in fines or continuous rejections at the vehicle inspection station.

The truth is that Keeping the mechanics up-to-date and in good condition is much more efficient than any dubious trick. A well-maintained engine, with clean filters, the right oil, properly functioning fuel injection, and operational emissions control systems, consumes less fuel, runs better, and has a better chance of passing the emissions test on the first try without any surprises.

The role of workshops and decarbonization in passing the MOT test

For many drivers, the garage has become an essential ally before going to the vehicle inspection (ITV). It's increasingly common to request a... pre-MOT check in which the professional checks those points that cause the most failures: lights, tires, brakes, suspension and, of course, emissions.

In the field of gases, workshops today have specific tools for clean and optimize the combustion and exhaust systemOne example is decarbonizing machines, which allow preventive maintenance not only of the engine itself, but also of components such as the EGR, the turbo or the particulate filter, reducing the accumulation of carbon deposits.

Teams like a hydrogen decarbonizer or internal cleaning systems They help eliminate residues that accumulate over time in combustion chambers and exhaust gas passages. The result is usually improved engine smoothness, a slight decrease in fuel consumption, and, above all, a significant reduction in smoke emissions in the test.

In addition to this, there are specific products, such as Premium fuel injector cleaners for diesel and gasoline enginesThese treatments are added to the fuel and work on deposits adhered to injectors, fuel lines, and combustion chambers. In modern common rail systems, these treatments can be especially useful for removing stubborn dirt and improving fuel atomization.

Data from industry associations directly link increased monitoring at service stations with increased work for repair shops, which see how Rejections due to emissions and other serious defects result in additional repairs and maintenance.In the long run, this translates into more refined vehicles, fewer serious breakdowns, and an overall improvement in safety and the air we breathe.

All this effort has a direct impact on the environment. Studies from Spanish universities have concluded that The vehicle inspection (ITV) prevents hundreds of thousands of vehicles with emissions far exceeding permitted levels from circulating each year.cutting tens of thousands of tons of harmful particles and reducing premature deaths related to air pollution by several hundred.

Road safety, environment and driver responsibility

Some drivers still see the vehicle inspection as a mere nuisance or a hidden tax. However, the data shows that the vehicle inspection is a A real filter that removes unsafe and highly polluting vehicles from circulationHeadlights that don't work, unbalanced brakes, worn-out tires, or out-of-control emissions are more than just a bad sign: they are a direct risk on the road.

Annual accident reports and industry studies indicate that, thanks to regular inspections, a considerable number of accidents are avoided. serious accidents, injuries and fatalities which would otherwise be linked to technical vehicle failures. The same applies to illnesses resulting from breathing air with high levels of particles and harmful gases generated by cars and heavy vehicles in poor condition.

In the case of emissions, the impact is twofold: on the one hand, public health is protected by reducing NOx, particulate matter and other pollutantsOn the one hand, it promotes the renewal and proper maintenance of the vehicle fleet. On the other, as vehicle inspection limits become stricter, it effectively forces drivers to... The most neglected or very old cars should go to the workshop or be taken out of circulation. if they cannot adapt.

A clear difference in rejection rates is also observed between vehicle types. Statistics indicate that vans and trucks over 3.500 kg They have the worst approval rates, with nearly 30% failing the first inspection, followed by buses and coaches, and vans and light trucks. In contrast, the agricultural vehicles, motorcycles and emergency services They tend to show better levels of preservation.

In any case, the responsibility lies with the driver and the vehicle owner. Having regular maintenance done, paying attention to warning lights, listening for unusual noises, checking tires and brakes periodically, and not letting the car become a patchwork of repairs is the best way to... Arrive at the vehicle inspection station with confidence and without fear of failing due to emissions or other serious defects..

If you understand the technical inspection for what it truly is—a tool to protect your safety, the safety of others, and the environment—it becomes much easier to assume that Passing the MOT test is not about "cheating the system", but about driving a mechanically sound vehicle that respects the air we share.With reasonable maintenance and some planning before the appointment, most of the problems that currently fail so many inspections, especially those related to emissions, can be avoided without last-minute scares or surprises.