Diesel MOT Emissions Test: Why Diesels Fail and How to Pass

Since May 2018, the MOT test for diesel vehicles has become significantly stricter. The changes included tighter smoke opacity limits, mandatory DPF (diesel particulate filter) checks, and an automatic fail for any visible smoke from the exhaust. Understanding what the tester looks for — and how to prepare — can save you from an expensive failure.

1. The 2018 Emissions Changes

The key changes that affected diesel vehicles were:

• Stricter smoke opacity limits — The acceptable level of smoke was reduced significantly
• DPF check — If the car was originally fitted with a DPF, the tester must confirm it is present and has not been tampered with. A missing or modified DPF is an automatic major failure
• Visible smoke — Any visible smoke of any colour from the exhaust is now a major failure, regardless of the opacity reading
• Dashboard warning lights — Any illuminated emissions-related warning light (including the engine management light) is a major failure

2. What the Tester Checks

3. Why Diesels Fail: Common Causes

• Clogged DPF — The most common diesel-specific failure. DPFs trap soot particles and need to be burned off (regenerated) periodically. Short journeys prevent regeneration, causing the DPF to clog
• Short journey driving — If the car is mainly used for short trips around town, the exhaust never gets hot enough to regenerate the DPF, leading to progressive clogging
• DPF removal — Some owners have their DPF removed to avoid regeneration issues. This is illegal and an automatic MOT failure since 2018
• Faulty EGR valve — A stuck or failed exhaust gas recirculation (EGR) valve can increase emissions and trigger warning lights
• Worn injectors — Injectors that do not atomise fuel properly can cause excessive smoke

4. How to Prepare Your Diesel for the MOT

1. Motorway drive — 20–30 minutes at sustained speed (50–70mph) to regenerate the DPF
2. DPF cleaner additive — Products like Wynn’s DPF Cleaner or JLM DPF Cleaner can help reduce soot levels when added to the fuel tank before a long drive. Use these a few days before the MOT for best results
3. Check for warning lights — Any emissions-related dashboard light is an automatic fail. Address these before the test
4. Ensure the DPF light is off — If the DPF warning light has been appearing, the filter needs attention before the MOT
5. Use quality diesel — A tankful of premium diesel (e.g., Shell V-Power Diesel or BP Ultimate) before the test can help. Premium diesel has additives that clean injectors and improve combustion

5. DPF Replacement Costs

If your DPF is beyond saving (cleaning does not restore it), replacement is expensive:

Before replacing the DPF, try professional cleaning first. Specialist companies can remove the DPF and clean it using chemical or thermal processes at a fraction of the replacement cost.

6. The DPF Warning Light: What to Do

When the DPF warning light appears on your dashboard, it means the filter is becoming clogged and needs regeneration. Here is what to do:

• Immediately — Drive the car at 50–70mph for 20–30 minutes. This should trigger passive regeneration and the light should go off
• If the light stays on — Book a garage visit. The car may need a forced regeneration (using diagnostic software) or the DPF may need professional cleaning
• Do not ignore it — A clogged DPF can lead to engine damage if left unattended. The car may eventually go into limp mode (reduced power)

7. AdBlue and SCR Systems

Many newer diesels (Euro 6 onwards) use AdBlue — a urea solution injected into the exhaust to reduce nitrogen oxide (NOx) emissions. The SCR (Selective Catalytic Reduction) system is part of the emissions control.

• If the AdBlue tank runs empty, most cars will not restart once turned off
• AdBlue is available at most petrol stations and costs £1–£2 per litre
• The typical tank lasts 5,000–10,000 miles depending on driving style
• A dashboard warning appears when levels are low — do not ignore it
• An SCR system fault can trigger an emissions warning light, which is an automatic MOT fail

8. Should You Still Buy a Diesel?

Diesel still makes sense for certain drivers:

If you do mainly short trips around town, a petrol, hybrid, or electric vehicle is a better choice. Short journeys are the primary cause of DPF problems, and the emissions implications (including ULEZ charges in London and other cities) make diesel increasingly costly for urban drivers.

Final Thoughts

Diesel emissions failures are avoidable with the right preparation. The most important steps are regular motorway driving to keep the DPF healthy, addressing any warning lights promptly, and taking a 20–30 minute drive before the MOT to ensure the DPF is regenerated. If you are buying a diesel, check the MOT history for previous emissions advisories or failures — they can indicate a car that has been used for short trips and may have ongoing DPF issues.

This guide is for informational purposes. Always consult a qualified diesel specialist for specific vehicle concerns. DPF removal is illegal in the UK.