Diesel MOT Emissions Test: How to Pass in the UK
Car Owl
Published in English •
Summary
- Diesel emissions limits are strict: The MOT measures visible smoke from your exhaust using a smoke meter.
- A DPF delete is an instant fail: Removing or tampering with the diesel particulate filter is illegal.
- A good run before the test helps: Driving at speed for 20 minutes can clear soot and improve your chances. See our MOT guide for more tips.
If you drive a diesel car, the emissions test is one of the toughest parts of the MOT. Fail rates for diesels are higher than for petrol cars.
Here is what happens during the test and how to give your car the best chance of passing.
How the Diesel Emissions Test Works
The tester inserts a probe into your exhaust pipe. The engine is revved to a set speed while the machine measures the opacity of the smoke.
Opacity means how thick or dark the smoke is. It is measured in units called k values. The lower the reading, the cleaner your exhaust.
| Vehicle Age | Maximum Smoke Reading |
|---|---|
| Registered before 2008 | 3.0 k value (1/m) |
| Registered from 2008 onwards | 1.5 k value (1/m) |
| Turbocharged (pre-2008) | 3.0 k value (1/m) |
Cars with a diesel particulate filter (DPF) must produce very low emissions. If the DPF is blocked or removed, the readings will be far too high.
Why Diesel Cars Fail the Emissions Test
The most common reasons for diesel emission failures are:
- Blocked DPF: Short journeys cause soot to build up in the filter. It needs regular long drives to burn off.
- DPF removed: Some owners delete the DPF to avoid problems. This is illegal and causes an instant MOT fail.
- Faulty injectors: Worn injectors spray too much fuel, creating excess smoke.
- Turbo problems: A failing turbo can leak oil into the exhaust, creating blue smoke.
- EGR valve issues: A stuck EGR valve can increase emissions.
- Old or dirty engine oil: Using the wrong oil or not changing it can affect combustion.
How to Prepare Your Diesel for the MOT
Follow these steps before your test to improve your chances:
- Take a long drive: Drive on the motorway for 20-30 minutes before the test. This heats up the DPF and burns off soot.
- Use the right oil: Make sure your engine has the correct grade of diesel engine oil. Check your owner's manual.
- Check your DPF warning light: If it is on, get the DPF regenerated or cleaned before the test.
- Use a fuel additive: A diesel DPF cleaner additive can help reduce soot in the system.
- Get a service: Fresh oil, new filters, and clean injectors all help reduce emissions. See our car servicing guide.
Do not let the engine idle for a long time before the test. A cold, idling diesel produces more soot. Drive the car to the garage and let the tester test it while the engine is warm.
DPF Rules and the Law
Since February 2014, the MOT includes a check for the diesel particulate filter. If your car was built with a DPF, it must still have one fitted.
- Removing a DPF is illegal: It is an offence under the Road Vehicles (Construction and Use) Regulations.
- Garages can be fined: Any garage that removes a DPF can face prosecution.
- Visual check: The tester will look for the DPF. If it is missing, the car fails immediately.
What If Your Diesel Fails?
If your diesel fails the emissions test, you have a few options:
- Get the DPF cleaned: Professional cleaning can restore a blocked DPF.
- Fix faulty injectors: A mechanic can test and replace worn injectors.
- Replace the EGR valve: If it is stuck, a new one can bring emissions back in line.
- Retest for free: If you fix the fault and return to the same garage within 10 working days, the retest is free.
Should You Keep Your Diesel?
Diesel cars face tougher emissions rules each year. If your car keeps failing, it might be time to consider switching.
Check your car's value with our car value tool and explore options in our clean air zones guide.
Final Thoughts
The diesel emissions test does not need to be scary. A long drive, fresh oil, and a healthy DPF will get most cars through.
Check your MOT history with our free MOT tool and prepare early for a stress-free test.
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