Diesel MOT Failures: What You Need to Know

Can a Diesel Car Fail an MOT? The Definitive Guide

The dreaded MOT test. For many car owners, it's a necessary evil, a yearly hurdle that can bring both relief and a hefty bill. When it comes to diesel cars, there's often a lingering question: can a diesel car fail an MOT? The straightforward answer is a resounding yes. In fact, diesel vehicles can fail their MOT for a multitude of reasons, many of which are common to petrol cars, but some are specific to the unique nature of diesel engines. This article will delve deep into the common pitfalls that can lead to a diesel car failing its MOT, what to look out for, and how to maximise your chances of a first-time pass.

Understanding the MOT Test for Diesels

The MOT (Ministry of Transport) test is a mandatory annual inspection in the UK designed to ensure that vehicles are roadworthy, safe, and environmentally compliant. While the core principles apply to all vehicles, diesel cars face specific checks, particularly concerning their emissions. The test is divided into several areas, each with potential failure points:

• Brakes: As with any vehicle, worn brake pads, discs, or faulty brake lines will result in an MOT failure.
• Lights: All lights must be functional and the correct colour.
• Steering and Suspension: Loose steering components, worn ball joints, or damaged suspension parts are critical safety issues.
• Tyres: Tyres must have a minimum tread depth of 1.6mm and be free from major damage or bulges.
• Exhaust System: This is where diesel cars often encounter issues.
• Visibility: Windscreen damage, faulty wipers, or a blocked washer system can lead to a fail.
• Bodywork: Excessive corrosion or sharp edges that pose a danger to others can cause a failure.
• Emissions: This is a crucial area for diesel vehicles.

The Diesel Emissions Test: A Major Hurdle

Perhaps the most significant area where diesel cars can fall foul of the MOT is their emissions. Since 2014, diesel cars have undergone a more stringent emissions test. The primary objective is to measure the opacity of the exhaust smoke, which is a direct indicator of incomplete combustion and the presence of soot particles. This is carried out using a piece of equipment called a smokemeter.

During the test, the engine is run at specific RPMs, and the smokemeter measures the amount of light that is blocked by the exhaust fumes. There are strict limits on the permissible smoke density, measured in metres per absorption coefficient (m⁻¹). These limits vary depending on the age and type of the diesel vehicle.

What Causes High Diesel Emissions?

Several factors can contribute to a diesel car failing the emissions test:

• Clogged Diesel Particulate Filter (DPF): This is a very common culprit. The DPF is designed to trap soot particles. If it becomes too clogged, it can restrict exhaust flow, leading to poor performance and excessive smoke. In some cases, a blocked DPF can even cause the engine to enter a 'limp mode'. Many modern diesels have DPF regeneration cycles, but if these are not completed successfully due to short journeys or other issues, the DPF can become permanently blocked.
• Faulty EGR Valve (Exhaust Gas Recirculation): The EGR valve recirculates a small amount of exhaust gas back into the combustion chamber to reduce NOx emissions. If the EGR valve gets stuck open or closed, it can disrupt the combustion process, leading to increased soot production and potentially a failed emissions test.
• Injector Problems: Worn or faulty fuel injectors can lead to improper fuel atomisation, resulting in incomplete combustion and increased smoke.
• Turbocharger Issues: A failing turbocharger can cause oil to leak into the combustion chamber, which, when burned, produces blue smoke and can lead to a failed emissions test.
• Air Filter: A severely clogged air filter can restrict airflow to the engine, leading to an overly rich fuel mixture and increased soot.
• Glow Plugs: While less directly related to smoke, faulty glow plugs can affect cold starting and engine efficiency, which could indirectly contribute to emissions problems.
• Incorrect Fuel: While rare, using the wrong type of fuel can have catastrophic consequences for a diesel engine and will undoubtedly lead to an MOT failure.

Common MOT Failure Points for Diesel Cars (Beyond Emissions)

While emissions are a major concern, diesel cars can fail their MOT for many of the same reasons as petrol cars. Here are some other common reasons:

• Brake Wear: Worn brake pads and discs are a safety critical item and a guaranteed fail. Check for consistent braking performance and any unusual noises.
• Tyre Condition: Beyond tread depth, damaged or perished tyres (cracked sidewalls) will cause a fail. Ensure your tyres are the correct pressure and suitable for the vehicle.
• Lights: A burnt-out bulb, a cracked lens, or a light that is too dim or too bright can all lead to failure.
• Suspension Components: Worn shock absorbers, leaky suspension struts, or damaged wishbones are serious safety concerns.
• Exhaust Leaks: While the emissions test focuses on smoke, significant leaks in the exhaust system before the catalytic converter or DPF can also cause a fail due to noise or safety.
• Corrosion: Significant rust on structural parts of the vehicle, such as the chassis or suspension mounting points, is a common reason for failure. This is particularly important for older diesel vehicles.
• Windscreen Damage: A chip or crack in the windscreen that obstructs the driver's view, especially in the 'sweep area' of the windscreen wipers, will result in a fail.

Preparing Your Diesel Car for its MOT

Proactive maintenance is key to ensuring your diesel car passes its MOT. Here’s how you can prepare:

Pre-MOT Checks You Can Do Yourself

Before taking your car to the test centre, a quick walk-around can save you a lot of hassle:

• Check all lights: Turn on your headlights (dipped and main beam), fog lights, indicators, brake lights, and reverse lights. Get someone to help you check the brake lights if needed.
• Tyre Check: Inspect the tread depth of all tyres using a 20p coin. The Queen's head should not be visible. Check for any cuts, bulges, or damage to the sidewalls.
• Windscreen: Clean your windscreen thoroughly. Check for any chips or cracks in the driver's line of sight.
• Wipers and Washers: Ensure your wipers clear the screen effectively and that the washer fluid sprays correctly.
• Fluid Levels: Check your engine oil, coolant, and brake fluid levels. While not a direct MOT failure, low fluid levels can indicate underlying issues.
• Horn: Make sure the horn works and sounds correctly.
• Seatbelts: Check that all seatbelts are functioning correctly and retract smoothly.

Regular Maintenance is Crucial

The best way to avoid MOT failures is through regular, scheduled maintenance. This includes:

• Servicing: Adhering to your car's service schedule is paramount. This will involve oil changes, filter replacements (oil, air, fuel), and checks of key components.
• DPF Care: For modern diesels, ensure you allow for DPF regeneration cycles. This typically means occasional longer drives at consistent speeds (e.g., motorway journeys). If you only do short, stop-start journeys, the DPF may not get a chance to clear itself. Consider using a DPF cleaner additive if you suspect it's becoming clogged.
• Fuel System Cleaning: Periodically using a quality diesel fuel system cleaner can help keep injectors clean and improve fuel atomisation.
• EGR Valve Cleaning/Replacement: If you notice symptoms like rough idling or reduced power, your EGR valve might be the culprit. Cleaning or replacing it can resolve these issues.

What Happens If Your Diesel Car Fails?

If your car fails its MOT, you will be issued with a refusal of an MOT test certificate. The report will detail the reasons for failure and any 'advisories' – issues that are not yet severe enough to cause a fail but will need attention in the future. You can still drive your car if it has a valid MOT certificate, but if it fails, you can only drive it to a pre-arranged MOT test appointment or to a place of repair. Driving a vehicle that requires an MOT and doesn't have a valid certificate is illegal and can result in fines and penalty points.

You have a number of options after a failure:

• Repair and Re-test: You can have the necessary repairs carried out by the test centre or take it elsewhere for repairs and then have it re-tested. If the re-test is carried out within 10 working days at the same test centre, some items may only require a partial re-test.
• Alternative Test Centre: You can take your car to a different test centre for a full re-test.

Common Diesel MOT Myths Debunked

There are several misconceptions surrounding diesel MOTs. Let's address a few:

• Myth: Older diesels are exempt from the emissions test. While older vehicles (typically pre-1986) have different, less stringent emissions requirements, most modern diesels are subject to the smokemeter test.
• Myth: You can 'cheat' the emissions test by revving the engine hard before the test. While a brief burst of revving might clear some minor build-up, it won't fix underlying mechanical issues causing high emissions. In fact, excessive revving could potentially cause further damage.
• Myth: A DPF delete is a good way to avoid emissions issues. Removing a DPF is illegal and will result in an MOT failure if detected. It also has significant environmental implications.

Table: Common Diesel MOT Failure Points and Solutions

Frequently Asked Questions

Q1: My diesel car is smoking excessively, what should I do?

If your diesel car is smoking heavily (black smoke usually indicates soot from incomplete combustion), it's highly likely to fail the emissions test. You should have it inspected by a qualified mechanic. Common causes include a clogged DPF, issues with the EGR valve, or problems with the fuel injectors.

Q2: Can I pass the MOT if my DPF warning light is on?

No, if your DPF warning light is illuminated on your dashboard, it indicates a fault with the DPF system. This will almost certainly lead to an MOT failure, particularly during the emissions test. You must get the DPF system diagnosed and repaired before the MOT.

Q3: How often should I service my diesel car?

It's recommended to follow the manufacturer's service schedule. Typically, this is every 12 months or after a certain mileage (e.g., 10,000-20,000 miles), whichever comes first. Regular servicing is the best preventative measure against MOT failures.

Q4: What are the consequences of driving a diesel car with a faulty DPF?

Driving with a faulty or removed DPF can lead to reduced engine performance, increased fuel consumption, and potential engine damage. Crucially, it will result in an MOT failure and is illegal for road use due to environmental regulations.

Conclusion

In summary, a diesel car can absolutely fail an MOT, and the emissions test is a critical area where many come unstuck. Understanding the specific requirements for diesel vehicles, particularly concerning smoke opacity and the proper functioning of systems like the DPF and EGR valve, is essential. By maintaining your diesel vehicle diligently, addressing any warning lights promptly, and performing basic pre-MOT checks, you can significantly increase your chances of a successful first-time pass. Regular maintenance isn't just about passing the MOT; it's about ensuring your diesel car remains reliable, efficient, and safe for years to come.

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