Diesel MOT Failures: What You Need to Know

The annual MOT test is a crucial part of car ownership, ensuring your vehicle meets safety and environmental standards. For diesel vehicle owners, understanding potential failure points is key to avoiding unexpected costs and hassle. Since recent changes, particularly the introduction of checks on AdBlue systems and more stringent emissions testing, diesel cars can face new hurdles. This article delves into the common reasons why a diesel car might fail its MOT, with a particular focus on emissions and exhaust-related issues, and what you can do to rectify them.

AdBlue Systems and the MOT

Since May 2018, the functionality of the AdBlue system has become a mandatory part of the MOT test for diesel vehicles. But what exactly does AdBlue do? AdBlue is a fluid designed to treat exhaust gases, significantly reducing the expulsion of harmful pollutants like nitrogen oxides (NOx). It works by converting these harmful emissions into harmless nitrogen and water vapour through a process called Selective Catalytic Reduction (SCR). If your AdBlue system is tampered with, deactivated, or faulty, it can lead to an MOT failure. It's also worth noting that while AdBlue is generally safe to handle, it's advisable to wear gloves if you have sensitive skin and avoid ingestion or eye contact.

Exhaust Smoke: A Clear Sign of Trouble

The MOT test has always included checks for visible smoke from the exhaust. However, recent updates have made these checks even more critical for diesel vehicles. Cars or commercial vehicles that emit thick clouds of black smoke are a common sight, but this is now a definite MOT failure. The introduction of a tougher emissions test means that a diesel vehicle with a Diesel Particulate Filter (DPF) that 'emits smoke of any colour' will automatically fail. This is a significant change, as a functional DPF is designed to capture soot particles from the exhaust. Tampering with or removing the DPF is also an automatic failure. A DPF warning light illuminated on the dashboard is also an MOT failure point, as no dashboard warning lights should be illuminated during the test.

Understanding Different Smoke Colours

While black smoke is a primary concern for diesel MOTs, other smoke colours can also indicate underlying issues:

• Black Smoke: This typically signifies incomplete fuel combustion. It can be caused by several factors, including faulty injectors, a blocked fuel filter, a dirty air filter leading to insufficient oxygen, a malfunctioning EGR valve, or even a turbocharger issue. Essentially, it means the diesel fuel isn't burning completely.
• Blue Smoke: Blue smoke indicates that engine oil is being burned. This can happen due to worn piston rings, damaged valve guides, or simply an overfilled engine oil level. If you notice blue smoke, especially during acceleration, it's a strong sign of internal engine wear.
• White Smoke: While thin white smoke that dissipates quickly is usually just steam, thicker, persistent white smoke can be a sign of a more serious problem. It often means that the car's coolant is leaking into the engine, which can lead to severe engine damage if not addressed promptly. Low oil levels, contrary to some beliefs, do not cause white smoke; they are more likely to lead to blue smoke if oil enters the combustion chamber.

DPF Issues: A Common Failure Point

The Diesel Particulate Filter (DPF) has been a key component in reducing diesel emissions since its introduction. As mentioned, its removal or tampering will result in an MOT failure. Beyond physical removal, a DPF can become clogged with soot over time. If the DPF warning light illuminates on your dashboard, it’s a strong indication that the filter needs attention. While a clogged DPF doesn't automatically mean an MOT failure, if it leads to excessive smoke or if the warning light remains on, it certainly will. Regular driving on short journeys can contribute to DPF clogging, as the exhaust system doesn't get hot enough to regenerate (burn off) the trapped soot. Active regeneration or professional cleaning might be required.

What to Do If Your Diesel Fails its MOT for Emissions

If your diesel vehicle fails its MOT due to emissions or any other reason, you have a few options. Firstly, you'll need to have the necessary repairs carried out. Once the repairs are completed, the vehicle must be re-submitted for a second MOT test. In many cases, if you return the vehicle to the same garage for a retest within a specified period (often 10 working days), the retest can be free of charge. However, it's crucial to confirm this with the garage beforehand. Ignoring the failure and continuing to drive could lead to further issues and legal complications, especially if the car is deemed unsafe or overly polluting.

Buying a Car That Has Failed Emissions

It's generally advisable to avoid purchasing a car that has failed its emissions test, especially if the failure is significant or related to fundamental engine or exhaust system issues. In many regions, it is illegal to sell a vehicle that does not have a current, valid emissions certificate. If a vehicle has a history of failed emissions tests or if its last test was a long time ago, it's a significant red flag. You should always question the seller and, if possible, have the vehicle inspected by an independent mechanic before committing to a purchase.

Common MOT Emission Test Requirements for Diesels

To pass the MOT emissions test, your diesel vehicle must meet the following criteria:

• No Visible Smoke: As reiterated, any visible smoke coming from the exhaust is an automatic failure.
• Emissions Restrictions: The vehicle must not exceed the specific emissions limits set for diesel engines. These limits are checked using an exhaust gas analyser.
• DPF Integrity: The DPF must not have been tampered with or removed.

How to Fix Common Diesel Emission Issues

Addressing the root cause of excessive smoke is essential for passing the MOT and maintaining the health of your diesel engine:

• Black Smoke: Consult a specialist. Cleaning the air intake system, checking engine rings, inspecting the fuel supply, and using appropriate fuel additives can help. Ensuring a clean air filter is also vital for proper combustion.
• Blue Smoke: This often requires engine repair. Worn piston rings or damaged valve guides will need professional attention. Checking and correcting the engine oil level is also important.
• White Smoke: If persistent, this usually indicates a coolant leak into the engine. This is a serious issue that requires immediate diagnosis and repair by a qualified mechanic to prevent catastrophic engine failure.

AdBlue Top-Ups and Servicing

AdBlue is not typically part of a standard annual service. Due to varying mileage, journey types, and driving styles, you may need to top up your AdBlue fluid between scheduled services. Many modern vehicles will provide a warning light when the AdBlue level is low, giving you ample time to refill. It's important to use the correct AdBlue fluid and to refill it according to your vehicle manufacturer's instructions.

Summary of Diesel MOT Failure Points

Here's a quick rundown of key areas where diesel vehicles commonly fail their MOT, particularly concerning emissions:

Regular maintenance, addressing warning lights promptly, and understanding the function of systems like AdBlue and the DPF are crucial for ensuring your diesel vehicle passes its MOT test smoothly. By staying informed and proactive, you can avoid the stress and expense of an MOT failure.