How to reduce car emissions for your MOT
It’s frustrating when your car fails its MOT — but it’s even more frustrating when the issue is emissions-related and you’re not sure what to do next.
Whether your diesel failed its MOT on emissions, or you’re trying to get ahead and reduce your car's emissions before the test, this guide will walk you through what causes the problem and what you can do about it.
Why MOTs fail on emissions
The DVSA takes emissions seriously. MOT testers check if your car is producing too much pollution based on set limits for CO2, carbon monoxide, hydrocarbons, and particulates.
Common reasons for an emissions-related MOT fail include:
Dirty or clogged fuel injectors
Excess soot in the exhaust
A failing EGR (Exhaust Gas Recirculation) valve
Worn spark plugs or ignition issues
Faulty sensors (oxygen, MAF etc.)
Old engine oil
Poor fuel combustion
For diesel cars especially, high soot levels or visible smoke from the exhaust are instant fails.
My diesel car failed MOT on emissions — what now?
Diesel vehicles are held to tighter standards, and a diesel emissions fail usually means your car is:
Producing visible smoke
Emitting excessive NOx or particulates
Failing the ‘free acceleration’ test
If this happened to you, here’s what to do:
Get a full diagnostic test
This will pinpoint the issue — whether it’s a faulty DPF, worn injectors, or a sensor.Use a high-quality fuel additive
Fuel additives like Fuelboost Pro can help clean injectors, remove carbon build-up, and restore proper combustion — often leading to a pass on retest.Replace clogged filters
A blocked air filter or fuel filter reduces efficiency and increases emissions.Drive it ‘hard’ before the test
Known as an ‘Italian tune-up,’ taking your diesel on a 20-minute motorway drive at higher revs can help clear out soot and prepare it for retest.
How to reduce car emissions for MOT (and avoid a fail)
If your MOT is coming up and you want to avoid that dreaded red ‘fail’ notice, here are proactive steps to reduce your car's emissions:
1. Use a fuel system cleaner
Regularly using a multi-action fuel treatment before your MOT can reduce emissions significantly. Look for one that cleans injectors, emulsifies water, and improves combustion.
2. Check your oil
Old or degraded oil can increase emissions. If your oil looks black and thick, get it changed before the test.
3. Replace spark plugs and air filters
In petrol cars, poor ignition or restricted airflow causes incomplete combustion — a major emissions culprit.
4. Warm the engine before the test
A cold engine produces more emissions. Driving for 15–20 minutes before your MOT can help the engine operate efficiently.
5. Avoid short journeys beforehand
Lots of stop-start driving increases soot in diesels. A longer, uninterrupted drive helps burn off build-up.
Will fuel additives help me pass my MOT?
Yes — in many cases. Fuel additives don’t fix mechanical faults, but they can:
Clean injectors
Improve fuel combustion
Reduce soot and carbon deposits
Lower CO2 and NOx emissions
This makes them especially useful if your car is borderline on emissions or has been used for lots of short trips.
Final thoughts
If your diesel car failed its MOT on emissions, or you're looking to reduce your emissions before the test, don’t panic.
With the right steps — including diagnostics, basic maintenance, and possibly a fuel additive — most cars can pass on a retest without costly repairs.
And if you want a quick win? A packet of Fuelboost Pro and a good motorway drive could be your ticket to a cleaner, MOT-ready car.
FAQs
How can I reduce my car’s emissions quickly?
Use a fuel additive, change the oil, and take the car for a high-speed motorway drive to burn off build-up. These three steps alone can significantly reduce emissions before your MOT.
What fuel additive helps pass MOT emissions?
Look for a multi-action additive like Fuelboost Pro that cleans injectors, reduces carbon deposits, and improves combustion efficiency.
Why did my diesel car fail its MOT on emissions?
Most likely due to excess smoke, a blocked DPF, or high soot/NOx output. These issues are often caused by short trips, dirty filters, or poor fuel combustion.
Can a car fail MOT for CO2 emissions?
CO2 itself isn’t a fail marker, but high CO2 usually means the engine is running rich or inefficiently — which can lead to failing other parts of the emissions test like CO or hydrocarbons.
Will driving fast clean my DPF?
Yes, in many cases. A 15–30 minute drive at motorway speeds (higher revs) helps regenerate the DPF and burn off trapped soot — especially if it’s not completely blocked.
Is it expensive to fix emissions problems?
Not always. A fuel system clean and some basic maintenance can cost under £50 and still help you pass the retest — especially for borderline cases.
