Worried about your vehicle passing its MOT emissions test?
Excessive emissions are one of the common reasons vehicles fail their MOT, particularly older diesel cars. The good news is that there are several simple steps you can take to help reduce emissions and improve your chances of passing first time.
Visit our maintenance guidance hub for more practical guides on keeping your vehicle roadworthy and reducing running costs.
Here’s everything you need to know.
What Is the MOT Emissions Test?
As part of every MOT inspection, your vehicle’s emissions are tested to ensure they comply with legal environmental standards.
The exact test varies depending on the age, fuel type, and engine technology of your vehicle.
For petrol vehicles, the MOT emissions test measures levels of:
- Carbon monoxide (CO)
- Hydrocarbons (HC)
- Lambda readings (air-to-fuel ratio)
For diesel vehicles, the test primarily measures:
- Exhaust smoke opacity
- Visible emissions
Modern vehicles are fitted with emissions control systems designed to reduce harmful pollutants, helping to improve air quality and meet environmental regulations.
If your vehicle produces emissions above the permitted limits, it may fail its MOT.
Why Do Vehicles Fail Emissions Tests?
Common causes of emissions-related MOT failures include:
- Faulty oxygen sensors
- Dirty fuel injectors
- Worn spark plugs
- Clogged air filters
- Engine management faults
- Diesel Particulate Filter (DPF) issues
- Exhaust leaks
- Poor vehicle maintenance
Fortunately, many of these issues can be prevented with regular servicing and routine maintenance.
How to Reduce Your Car’s Emissions Before an MOT
Use Quality Fuel
While premium fuel isn’t a guaranteed solution, higher-quality fuels often contain additional cleaning additives that can help keep fuel injectors and combustion systems cleaner.
If your vehicle has been used mainly for short journeys, switching to a premium fuel for a few tanks before the MOT may help improve combustion efficiency.
However, premium fuel alone will not fix underlying mechanical issues.
Take Your Car for a Longer Drive
One of the most effective things you can do before an MOT is take the vehicle on a longer motorway or dual carriageway drive.
This is particularly important for diesel vehicles fitted with a Diesel Particulate Filter (DPF).
A sustained drive at higher engine temperatures can help:
- Burn off accumulated soot
- Assist DPF regeneration
- Reduce excessive smoke emissions
A 20–30 minute drive at motorway speeds before the test can often make a noticeable difference.
Check Your Diesel Particulate Filter (DPF)
Most diesel vehicles manufactured from around 2009 onwards are fitted with a DPF.
The DPF captures soot particles from the exhaust system and reduces harmful emissions.
If the filter becomes blocked, you may experience:
- Reduced performance
- Warning lights
- Increased emissions
- MOT failure
It’s important not to remove or tamper with a DPF.
Removing a DPF is illegal and will almost certainly result in an MOT failure.
If you suspect a blockage, seek professional advice before your test.
Replace Dirty Air Filters
A clogged air filter can restrict airflow into the engine and affect the fuel-air mixture.
Replacing a dirty air filter can:
- Improve fuel efficiency
- Improve combustion
- Reduce emissions
Air filters are inexpensive and often overlooked during routine maintenance.
Ensure Your Engine Is Serviced
A well-maintained engine typically produces lower emissions.
If your service is overdue, consider arranging:
- Oil and filter changes
- Spark plug replacement (petrol vehicles)
- Fuel filter replacement
- Engine diagnostics
Addressing small maintenance issues before your MOT can help avoid emissions failures.
Check Your Tyre Pressures
Tyres don’t directly affect the MOT emissions test, but underinflated tyres increase rolling resistance, forcing the engine to work harder and consume more fuel.
Maintaining the correct tyre pressure can help:
- Improve fuel economy
- Reduce overall emissions
- Improve vehicle safety
Check your tyre pressures regularly and adjust them according to the manufacturer’s recommendations.
Address Warning Lights
If your engine management light is illuminated, don’t ignore it.
The warning light may indicate issues affecting emissions systems, including:
- Oxygen sensors
- EGR valves
- DPF systems
- Catalytic converters
A diagnostic check can identify the problem before your MOT appointment.
And if you don’t know what your warning lights mean, check out our guide to dashboard warning lights explained where we reveal all.
Can Fuel Additives Help?
Fuel system cleaners and emissions-reducing additives may help clean deposits within the fuel system and improve combustion.
While they are not a guaranteed fix, some drivers find them beneficial when used alongside regular maintenance.
Always use products suitable for your vehicle and follow the manufacturer’s instructions.
The Importance of Regular Maintenance
The best way to reduce emissions isn’t just before an MOT—it’s all year round.
Regular servicing helps:
- Maintain engine efficiency
- Reduce fuel consumption
- Lower emissions
- Prevent costly repairs
- Improve your chances of passing future MOTs
Looking after your vehicle throughout the year is often far more effective than trying to fix problems shortly before the test.
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