Diesel MOT Test Changes Explained

The Diesel MOT Test: What's Really Changed?

For many diesel vehicle owners, the MOT test can be a source of anxiety, particularly concerning emissions. In recent years, there have been whispers and rumours about significant changes to the diesel MOT test, leading to confusion and uncertainty. This article aims to cut through the noise and provide a clear, comprehensive understanding of the current state of the diesel MOT, what to expect, and how to ensure your vehicle passes.

Understanding the Emissions Test for Diesels

The core of the MOT test for diesel vehicles revolves around ensuring they meet stringent emissions standards. This is primarily achieved through a check of the exhaust emissions. For diesels, this involves a diesel particulate matter (DPM) test, often referred to as the 'plume' or 'smoke' test. This test measures the opacity of the exhaust smoke, which is a good indicator of the efficiency of the combustion process and the presence of unburnt fuel or soot.

The test is conducted using a piece of equipment called an opacimeter. The vehicle is placed on a rolling road (dyno) and run up to a specified engine speed. While at this speed, the engine is suddenly released, and the opacimeter measures the density or opacity of the smoke emitted from the exhaust pipe for a set period. A higher opacity reading indicates more smoke, suggesting potential issues with the engine, fuel system, or exhaust after-treatment system.

Debunking the Myths: What Hasn't Changed?

It's important to address some common misconceptions. Despite what you might hear, there haven't been any wholesale, radical overhauls of the *fundamental* diesel MOT emissions testing procedure in the UK in the last few years. The core principles of checking for visible smoke and ensuring the vehicle's emissions control systems are functioning correctly remain the same. For instance, the requirement for a catalytic converter or a diesel particulate filter (DPF) to be fitted and functioning has been a part of MOT regulations for a considerable time.

What *has* evolved are the advisory notes and the interpretation of results. The Driver and Vehicle Standards Agency (DVSA), the body responsible for MOT testing, continually refines its guidelines to improve road safety and environmental protection. This means that while the test method might be similar, the thresholds and the emphasis placed on certain aspects can subtly change over time.

The Role of the Diesel Particulate Filter (DPF)

The Diesel Particulate Filter (DPF) is a crucial component in modern diesel vehicles designed to capture harmful soot particles from the exhaust gases. The MOT test explicitly checks for the presence and functionality of the DPF.

• Mandatory Presence: If a vehicle was originally fitted with a DPF, it must be present and correctly fitted during the MOT test. Removing or tampering with a DPF is a major failure.
• DPF Functionality: While the MOT test doesn't directly measure the DPF's 'cleanliness' or regeneration cycle, a malfunctioning DPF can lead to increased smoke, which will be picked up by the opacity test. Furthermore, a warning light for the DPF system illuminated on the dashboard will also result in a failure.

What About AdBlue and SCR Systems?

Selective Catalytic Reduction (SCR) systems, which use AdBlue to reduce nitrogen oxide (NOx) emissions, are also becoming increasingly common in diesel vehicles. While the MOT test doesn't directly test the AdBlue fluid itself, the system's functionality is indirectly assessed.

• Warning Lights: If the AdBlue warning light is illuminated on the dashboard, indicating a fault with the SCR system, the vehicle will fail its MOT.
• Tampering: Similar to the DPF, any evidence of tampering with or removal of the SCR system will lead to an immediate failure.

Common Reasons for Diesel MOT Failure (Emissions Related)

Understanding why a diesel vehicle might fail its emissions test can help owners prevent issues:

Preparing Your Diesel for its MOT

Proactive maintenance is key to a successful MOT. Here are some tips:

1. Regular Servicing: Ensure your vehicle is serviced according to the manufacturer's recommendations. This includes checking and replacing air filters, fuel filters, and oil filters.
2. DPF Maintenance: If your vehicle has a DPF, ensure it's being used correctly. Short, infrequent journeys can prevent the DPF from regenerating properly, leading to blockages. Regular longer drives at consistent speeds can help. Consider professional DPF cleaning if you suspect issues.
3. AdBlue Levels: For vehicles with SCR, always ensure the AdBlue tank is topped up.
4. Check for Warning Lights: Before your MOT, make sure no engine management lights, DPF warning lights, or AdBlue warning lights are illuminated on your dashboard.
5. Fuel Quality: Use good quality diesel fuel from reputable suppliers.
6. Engine Condition: A well-maintained engine is less likely to produce excessive smoke. Address any engine performance issues promptly.

FAQs About the Diesel MOT Test

Q1: Has the MOT test started checking for engine remapping or 'chip tuning' on diesels?

A1: While the MOT test doesn't directly check for engine remapping in the same way it checks emissions, any remapping that significantly alters emissions characteristics, such as disabling the EGR or DPF, can lead to a failure if it results in excessive smoke or illuminated warning lights. The DVSA is increasingly aware of these modifications.

Q2: What is the 'plume' test?

A2: The 'plume' test is the common name for the diesel emissions opacity test, where an opacimeter measures the density of smoke emitted from the exhaust under load.

Q3: If my DPF is missing, can I just fit a blanking pipe?

A3: No. If your vehicle originally came with a DPF, it must be present and functional. Fitting a blanking pipe or removing the DPF is a serious offence and will result in a major failure of the MOT test.

Q4: My car is old, does it have different emissions standards for the MOT?

A4: Yes, older vehicles (typically pre-1997 petrol, and specific diesel categories) have different testing procedures and standards. However, for modern diesels, the emissions testing is rigorous.

Q5: What if my car fails the emissions test?

A5: If your vehicle fails the emissions test, you will receive a refusal certificate. You'll need to have the necessary repairs carried out and then have the vehicle re-tested. You usually have 10 working days to get the repairs done and re-tested at the same garage without paying the full test fee again.

Conclusion: Stay Informed and Prepared

The diesel MOT test remains a vital check to ensure vehicles are not emitting excessive pollutants. While there haven't been dramatic recent changes to the core testing methodology, the emphasis on emissions control systems like the DPF and SCR, alongside the interpretation of results, continues to evolve. By understanding these requirements and maintaining your diesel vehicle diligently, you can significantly increase its chances of passing the MOT test and contribute to cleaner air on our roads. Regular servicing, proper DPF care, and addressing any warning lights promptly are your best defence against an MOT failure.

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