Failed Diesel MOT? Your Guide to Emissions

That familiar knot of anxiety tightens as you await the verdict of your vehicle’s MOT inspection. For many diesel car owners, especially those with older models, this feeling has become all too common since May 2018. That’s when the MOT test underwent significant updates, introducing much tighter regulations for diesel emissions. The result? A dramatic increase in failures, particularly related to tailpipe emissions and the functionality of the engine management system, with a laser focus on the Diesel Particulate Filter (DPF).

Since these changes, the DPF has become a central point of scrutiny during any MOT. The statistics speak volumes: between May 2018 and February 2019, there was a staggering 63% increase in emissions-related failures for diesel vehicles compared to the preceding period. While your car might have seemed to be running perfectly fine, a glowing dash light signalling a DPF issue was often the first alert for the MOT tester, instantly flagging your vehicle for a problem with its emissions control system and, inevitably, a failure.

The Big MOT Shake-Up: What Changed for Diesels?

Before May 2018, the diesel emissions test primarily involved a smoke opacity test. While this checked for excessive smoke, it wasn't as stringent in detecting subtle issues or problems with the DPF system itself. The 2018 overhaul brought a more comprehensive approach, targeting several key areas:

• Stricter Smoke Limits: While still a smoke test, the acceptable limits for opacity were tightened, making it harder for vehicles with even minor issues to pass.
• Visual DPF Inspection: Testers now conduct a thorough visual inspection of the DPF. Any evidence of tampering, removal, or damage will result in an immediate fail. This is crucial because some owners previously removed DPFs to avoid costly replacements, a practice now illegal and easily detectable.
• Engine Management Light (EML) Scrutiny: Perhaps the most impactful change for DPFs is the automatic fail if the DPF warning light (or any engine management light indicating an emissions fault) is illuminated on the dashboard. This means even if the vehicle isn't visibly smoking, a warning light is enough to fail.
• Diesel Particulate Filter (DPF) Functionality: The test now specifically looks for issues that indicate a malfunctioning DPF, such as an incomplete regeneration cycle or excessive soot loading.

These changes were implemented to ensure diesel vehicles meet modern emission standards, reducing harmful pollutants released into the atmosphere. For older diesel cars, which were not originally designed with such stringent DPF monitoring in mind, adapting to these new rules has proven to be a significant challenge.

Why Are Diesels Failing Emissions More? The DPF Culprits

The single biggest reason for these increased MOT failures is DPFs clogged with excessive soot and ash, which in turn triggers the engine management light. Understanding why this happens is the first step towards a solution. There are five primary causes for a DPF warning light to illuminate, listed here in their order of common occurrence:

1. Short Vehicle Journeys: This is by far the most common cause. DPFs need to reach high temperatures (typically through sustained driving at higher speeds) to perform a self-cleaning process called regeneration. If your driving consists mainly of short trips around town, the DPF never gets hot enough to complete this cycle, leading to soot accumulation.
2. Faulty or Blocked Fuel Injectors: Injectors that aren't atomising fuel correctly can lead to incomplete combustion. This results in more unburnt fuel particles entering the exhaust system, turning into excessive soot that clogs the DPF faster than it can regenerate.
3. Incorrect Engine Oil Used: Not all engine oils are created equal for modern diesels. Using an engine oil that isn't specifically formulated for vehicles with DPFs (often labelled as 'low ash' or Low SAPS – Sulphated Ash, Phosphorus, and Sulphur) can lead to ash build-up in the DPF. Unlike soot, ash cannot be burned off during regeneration and permanently reduces the DPF's capacity.
4. Faulty Turbocharger: A malfunctioning turbocharger can cause excessive oil consumption, leading to oil entering the exhaust system. This oil burns and creates ash, which, like the incorrect engine oil, can clog the DPF permanently.
5. Faulty EGR Valves: The Exhaust Gas Recirculation (EGR) valve helps reduce NOx emissions by recirculating some exhaust gases back into the engine. If the EGR valve is stuck open or closed, it can affect combustion efficiency, leading to increased soot production and DPF clogging.

My Diesel Failed the MOT Emissions Test – Now What?

Receiving that 'Failed' slip can be disheartening, but it's not the end of the road. Your next steps depend on the root cause of the failure. For issues like a faulty turbocharger or EGR valve, there's little you can do yourself; a qualified mechanic will be needed to diagnose and replace the faulty parts. However, the good news is that if your vehicle failed due to the first three common issues – short journeys, faulty injectors, or incorrect oil – you often have a significant chance to rectify the problem yourself, potentially saving a considerable sum on garage bills.

Addressing the Common Culprits: Oil and Fuel Treatments

1. Engine Oil Matters: The Low SAPS Requirement

Before you consider anything else, check your engine oil. If you've recently had an oil change, ensure the correct 'low ash' or Low SAPS oil was used. If you're unsure, or if it's due for a change, switching to the correct specification oil is a vital, inexpensive first step. This prevents further ash build-up and allows the DPF to function as intended. Always refer to your vehicle's manufacturer specifications for the precise oil type.

2. Getting the DPF to Regenerate: The Power of Fuel Treatment

Once the oil is sorted (or if you know it's already correct), the next focus is on enabling the DPF to perform a complete regeneration cycle. This is where a high-quality fuel treatment can make a significant difference. We recommend a product like Dipetane fuel treatment, which is a 100% hydrocarbon liquid fuel, fundamentally different from typical fuel additives.

Dipetane works by promoting a more complete burn in the combustion chamber. This has a multi-faceted benefit:

• Cleans Injectors: By ensuring a more thorough burn, Dipetane helps to clean and maintain the efficiency of your fuel injectors, reducing the likelihood of incomplete combustion and soot formation.
• Higher Combustion Temperatures: The more complete burn leads to higher temperatures within the combustion chamber and, consequently, in the exhaust system. This is crucial for the DPF.
• Hotter DPF Temperatures: With increased exhaust temperatures, the DPF itself gets hotter, creating the ideal conditions for the trapped soot and carbon to oxidise and be converted into harmless ash and gas during the regeneration cycle.

How to Use Dipetane for DPF Regeneration:

The standard mix is 1:200, but for an initial treatment after an MOT failure, it's advisable to add a little extra. Follow the specific measurements on the side of the bottle for subsequent uses. Once added to a sufficient amount of fuel in your tank, the crucial next step is to take your vehicle for an hour-long highway drive. This sustained, higher-speed driving allows the engine to reach and maintain the necessary temperatures for a full DPF regeneration cycle. It's important to understand that this process may not be instant; it could take a few days of repeated longer journeys for the treatment to fully work and for the DPF to clear sufficiently. Drive your car on as many long-distance journeys as possible during this period.

What if the Home Treatment Isn't Enough? The Garage-Assisted Boost:

If, after several long drives with Dipetane, your DPF warning light persists or you still fail the emissions retest, your friendly garage can provide a more intensive treatment. They should remove the fuel filter and fill it directly with Dipetane. This delivers a concentrated dose, causing an even larger, hotter burn in the combustion chamber almost immediately. After this, it's critical for the garage to set the vehicle into DPF regeneration mode using diagnostic equipment. Once this is complete, the DPF fault code should be cleared from the engine control unit (ECU), and the emissions retested. This method offers your best chance of passing your MOT without the significant expense of a professional DPF clean or, worse, a full DPF replacement.

Comparative Solutions for DPF Issues

Here's a quick look at common DPF issues and potential solutions:

Prevention is Better Than Cure: Maintaining DPF Health

As the old saying goes, prevention is often better than cure. Once you've successfully passed your MOT or addressed a DPF issue, adopting a proactive maintenance routine can help you avoid future headaches and expensive repairs. By consistently using a product like Dipetane regularly – perhaps one bottle in a tank of fuel each month – and ensuring you go for at least one sustained long drive, you can prevent DPF issues from recurring altogether.

Regular use of Dipetane will keep your fuel injectors working cleanly, ensuring a more efficient and complete burn in the combustion chamber. This, in turn, guarantees that your DPF consistently reaches the optimal temperatures required to carry out a full regeneration of any soot and ash stored within it. This simple routine can save you significant time, money, and stress in the long run, ensuring your diesel vehicle remains compliant with MOT emissions standards and runs efficiently for years to come.

Frequently Asked Questions (FAQs) About Diesel MOT and DPFs

What exactly is a Diesel Particulate Filter (DPF)?

A DPF is a ceramic filter located in the exhaust system of diesel vehicles, designed to trap harmful soot and particulate matter from the exhaust gases before they are released into the atmosphere. It's a crucial component for meeting modern emission standards.

Why do DPFs get blocked?

DPFs get blocked when the soot trapped inside cannot be burned off. This often happens due to a lack of complete regeneration cycles (e.g., from frequent short journeys), faulty engine components leading to excessive soot production, or the use of incorrect engine oil that leaves non-burnable ash deposits.

Can I drive with a DPF warning light on?

It is highly inadvisable to drive with a DPF warning light illuminated. It indicates a problem that needs immediate attention. Continued driving can lead to a severely blocked DPF, potentially causing permanent damage that requires expensive replacement. It will also cause your vehicle to fail its MOT.

How often should I use fuel treatment like Dipetane?

For preventative maintenance, using Dipetane once a month with a tank of fuel is recommended, combined with regular longer drives. If you're addressing an existing issue, follow the initial treatment instructions more closely, potentially requiring more frequent use until the issue resolves.

What is DPF regeneration?

DPF regeneration is the process by which the accumulated soot inside the filter is burned off. This can be passive (occurring automatically during high-speed driving when exhaust temperatures are high) or active (when the engine's ECU injects extra fuel to raise exhaust temperatures for a burn-off cycle). Forced regeneration can also be initiated by a mechanic using diagnostic tools.

Is DPF cleaning necessary, or can fuel treatments replace it?

Fuel treatments like Dipetane can prevent DPF blockages and often resolve early-stage issues by promoting more efficient regeneration. However, if a DPF is severely clogged with stubborn soot or ash deposits that cannot be burned off, a professional DPF clean (where the filter is physically removed and cleaned) or even replacement might be necessary. Fuel treatments are excellent for prevention and early intervention.

What does 'low ash' or 'Low SAPS' engine oil mean?

Low SAPS stands for Sulphated Ash, Phosphorus, and Sulphur. These are elements found in some engine oils that, when burned, can leave behind ash deposits that permanently clog DPFs. 'Low ash' or Low SAPS oils are specifically formulated with reduced levels of these elements to protect the DPF and ensure its longevity. Using the correct oil is crucial for DPF-equipped vehicles.

If you want to read more articles similar to Failed Diesel MOT? Your Guide to Emissions, you can visit the Maintenance category.