MOT Emissions: Petrol vs. Diesel Failures

The annual MOT test is a crucial hurdle for every vehicle on UK roads, ensuring it meets minimum safety and environmental standards. For many years, the emissions check has been a significant component of this assessment, but since May 2018, the rules have become considerably stricter. This tightening of regulations has led to a dramatic surge in emissions-related failures, leaving many motorists wondering which type of vehicle – petrol or diesel – is more likely to fall foul of the updated test.

While recent data reveals a staggering increase in diesel car failures, the complete picture is more nuanced. Understanding the changes, the specific challenges faced by each fuel type, and how to properly prepare your vehicle can save you time, money, and the frustration of an MOT retest.

The Stricter MOT Rules of 2018: A Game Changer

The landscape of the MOT test shifted significantly in May 2018 with the introduction of tougher emissions checks. These changes were not arbitrary; they came as a direct response to growing concerns over air quality, exacerbated by events such as the Volkswagen 'Dieselgate' scandal in 2015. The government sought to crack down on vehicles contributing excessively to harmful pollutants, particularly those with modified or malfunctioning emission control systems.

Key updates included:

• Diesel Particulate Filter (DPF) Checks: Any diesel car equipped with a DPF from new will now automatically fail its MOT if there is evidence the part has been tampered with or removed. It is now illegal to remove a DPF from a car that was originally fitted with one.
• Visual Smoke Test: A mechanic can instantly refuse to pass a car if they witness smoke of any colour coming out of the exhaust pipe. This is a clear indicator of an emissions problem.
• New Fault Categories: The DVSA introduced new fault categories – ‘Major’ and ‘Dangerous’. Emissions issues can result in either of these, leading to an automatic failure. A ‘Dangerous’ fault means the car should not be driven until the issue is rectified, posing an immediate risk to roadworthiness.

These stringent measures were designed to ensure that vehicles comply with the latest Euro emissions standards, with DPFs becoming standard on all diesel cars from 2009 to meet Euro 5 requirements. The previous practice of drivers having DPFs removed due to clogging issues, which reduced vehicle performance, was a primary target of these new rules.

Diesel Cars: The Dramatic Emissions Surge

The impact of the 2018 MOT changes has been most profoundly felt by diesel car owners. Data obtained through a Freedom of Information (FOI) Act request to the DVSA by BookMyGarage.com highlights a dramatic spike in diesel emissions failures. Between 2017 and 2018, just 123,596 diesel cars failed due to emissions. However, in the following year (2018-2019), this figure surged to an astonishing 397,991, representing a 240% increase compared to pre-2018 levels.

This upward trend continued into the 2019-2020 financial year, with over 420,537 diesel cars failing their MOTs on emissions. The primary culprit behind this significant increase is the DPF. These filters are designed to capture harmful soot particles from diesel exhaust gases, preventing them from entering the atmosphere. When DPFs become clogged, or worse, are tampered with or removed, the vehicle's emissions drastically increase, leading to an automatic MOT failure under the new rules.

Furthermore, a notable proportion of diesel emissions failures – approximately five per cent – were categorised as ‘Dangerous’ faults. This classification means the vehicle is deemed unsafe to drive until the problem is addressed, underscoring the severity of these particular emissions issues.

The ‘Dieselgate’ scandal undoubtedly cast a long shadow over diesel vehicles, contributing to a significant decline in their market share. In 2015, diesel cars accounted for around half of all new car sales in the UK; by last year, this figure had plummeted to just 16 per cent. This shift in public perception and stricter regulations have undeniably made MOTs a tougher proposition for many diesel owners.

Petrol Cars: Still Leading in Raw Numbers

While the percentage increase in diesel failures has been monumental, it's crucial not to overlook the persistent challenge faced by petrol cars. Despite the dramatic surge in diesel failures, petrol vehicles still account for a higher *total number* of emissions-related MOT failures annually. In the 2019-2020 financial year, almost 850,000 petrol vehicles were automatically failed in their annual check-up because of emissions issues.

Looking at the raw numbers, petrol car failures consistently outnumber diesel failures, even after the 2018 rule changes. In 2018-2019, 910,620 petrol cars failed compared to 397,991 diesels. The following year, it was 849,740 petrol failures versus 420,537 diesel failures. This indicates that while diesels have seen a more rapid *rate* of increase, the sheer volume of petrol cars on the road means they still contribute more to the overall emissions failure count.

Most petrol emissions failures are typically classified as ‘Major’ faults, meaning they require repair but generally do not deem the vehicle immediately ‘Dangerous’ to drive in the same way some diesel faults do. Common causes for petrol emissions failures include faulty catalytic converters, oxygen sensors, spark plugs, or general engine management issues that lead to inefficient combustion and higher levels of pollutants like carbon monoxide and unburnt hydrocarbons.

The Nuance of Failure Rates: A Deeper Dive

So, are petrol cars more likely to fail their MOT test due to emissions, or are diesels? The answer is nuanced and depends on how you look at the data. Diesels have experienced a far greater *surge* in failure rates since the 2018 rules, with a 240% increase. However, when considering the total number of failures, petrol cars consistently have more. Furthermore, if we look at the percentage of total licensed cars failing annually due to emissions, petrol cars still have a higher rate: 4.5 per cent for petrol compared to 3.3 per cent for diesels. This suggests that a higher proportion of petrol cars on the road are failing their emissions test each year.

Here's a breakdown of MOT emissions failures by fuel type, illustrating the shifts:

*New MOT regulations introduced from 20th May 2018. Source: FOI request to DVSA by BookMyGarage.com.

The table clearly shows the dramatic jump in diesel failures post-2018, but also the consistently higher volume of petrol failures. The primary takeaway is that while diesel cars face a new, tougher challenge due to DPF regulations, petrol cars continue to be a significant contributor to overall emissions failures.

Why the Difference?

The discrepancy in failure patterns can be attributed to several factors:

• Volume of Vehicles: There are simply more petrol cars on UK roads than diesels, historically. Even with the decline in new diesel sales, the existing fleet of petrol vehicles is larger, naturally leading to more raw failures.
• Nature of Emissions: Diesel engines produce different types of pollutants (primarily particulate matter and nitrogen oxides) compared to petrol engines (carbon monoxide, unburnt hydrocarbons, nitrogen oxides). The DPF is a specific technology addressing diesel particulates, and its strict enforcement has directly impacted diesel failure rates.
• DPF Vulnerability: DPFs can become clogged, especially if the car is used predominantly for short journeys where the filter doesn't get hot enough to regenerate. This, combined with the illegality of removal and the harsh penalties for tampering, makes the DPF a critical failure point for diesels.
• Petrol Emissions Causes: Petrol car emissions failures often stem from issues with catalytic converters (which degrade over time), oxygen sensors (which monitor exhaust gases), or general engine wear and tear that affects combustion efficiency. These are common maintenance issues across the fleet.

Tips for a Smooth MOT Emissions Pass

Regardless of whether you drive a petrol or diesel car, proactive maintenance is your best defence against an emissions-related MOT failure. Here are some key tips:

• Regular Servicing: Adhere to your car's service schedule. A well-maintained engine is an efficient engine, producing fewer harmful emissions. This includes oil changes, filter replacements, and spark plug checks.
• Engine Management Light (EML): Never ignore a lit EML. This light often indicates a fault with the engine management system, which can directly impact emissions. Get it diagnosed by a qualified mechanic immediately.
• For Diesel Owners – DPF Care: If you own a diesel with a DPF, ensure you regularly perform 'regeneration' drives. This involves driving at a sustained speed (e.g., on a motorway) for at least 20-30 minutes to allow the DPF to heat up and burn off trapped soot. If your DPF warning light comes on, do not ignore it.
• Check for Smoke: Before your MOT, start your car and look for any coloured smoke from the exhaust. Blue smoke often indicates burning oil, black smoke suggests unburnt fuel (common in diesels), and white smoke can indicate coolant issues. Any of these could lead to a fail.
• Fuel Quality: Use good quality fuel from reputable stations. Poor quality fuel can lead to deposits and inefficient combustion.
• Take a Longer Drive Before the Test: A good pre-MOT drive, especially for diesel cars, can help ensure the catalytic converter or DPF is at its optimal operating temperature, allowing it to perform effectively during the test.

Frequently Asked Questions About MOT Emissions

Here are some common questions drivers have regarding MOT emissions checks:

What is a Diesel Particulate Filter (DPF)?

A DPF is a ceramic filter fitted into the exhaust system of diesel vehicles. Its purpose is to capture harmful soot (particulate matter) from exhaust gases, preventing it from being released into the atmosphere. Over time, the trapped soot needs to be burned off (a process called regeneration) to clear the filter.

Is it illegal to remove a DPF from my car?

Yes, it is now illegal to remove a DPF from a car that was originally fitted with one by the manufacturer. If evidence of DPF tampering or removal is found during an MOT, the vehicle will automatically fail.

What happens if my car fails on emissions?

If your car fails its MOT due to emissions, you will be issued a refusal of MOT certificate. You cannot legally drive the car on the road until the fault is rectified and it passes a retest, unless you are driving it to a pre-booked repair appointment or to a pre-booked MOT retest. If the fault is classified as ‘Dangerous’, you should not drive the car at all.

Can I drive with a ‘Dangerous’ emissions fault?

No. If your car receives a ‘Dangerous’ fault for emissions (or any other component), it means the vehicle poses an immediate risk to road safety or the environment. It is illegal and highly unsafe to drive a vehicle with a dangerous fault until it has been properly repaired and passed its MOT retest.

How can I prepare my car for the emissions test specifically?

Beyond general maintenance, for the emissions test, ensure your engine is thoroughly warmed up before the test. This helps the catalytic converter and DPF operate efficiently. Consider adding a fuel system cleaner to your tank a few weeks before the test, and avoid driving with a low fuel level. If you have a diesel, ensure you've done a good motorway run to allow for DPF regeneration.

Conclusion

The stricter MOT emissions rules introduced in 2018 have undeniably created a more challenging environment for both petrol and diesel vehicle owners. While diesel cars have experienced a phenomenal surge in failure rates due to the rigorous DPF checks, leading to a much higher *percentage increase* in fails, petrol cars still account for a greater *total number* of emissions failures annually, and a higher *proportion* of licensed cars failing. This highlights that emissions compliance is a universal challenge, not exclusive to one fuel type.

Ultimately, the key to navigating the MOT emissions test successfully lies in diligent vehicle maintenance and understanding the specific requirements for your car's fuel type. Regular servicing, addressing warning lights promptly, and ensuring your car's emission control systems are in good working order are the best ways to keep your vehicle on the road and contribute to cleaner air for everyone.

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