Emissions failures are a common reason cars fail their MOT in the UK, particularly for older petrol and diesel vehicles. As MOT day approaches, many drivers consider using fuel additives to improve engine performance and reduce exhaust emissions. But do fuel additives really help, or are they just a temporary fix?
This article explains how fuel additives can affect MOT emission results, when they may help, and when they are unlikely to make a difference.
The MOT emissions test checks whether your vehicle produces harmful pollutants within legal limits. The specific checks depend on fuel type and vehicle age.
For petrol cars, testers measure:
For diesel vehicles, the test focuses on:
If emissions exceed permitted limits, the vehicle will fail the MOT regardless of overall engine performance.
Fuel additives are chemical products added to petrol or diesel to clean internal engine components and improve combustion. Common claims include:
Most additives work gradually and are designed to support engine health rather than instantly fix faults.
In some cases, fuel additives can improve emissions enough to pass an MOT, particularly when issues are minor.
They may help if:
Using an additive alongside a long motorway drive before the MOT can help burn off deposits and stabilise emissions.
Fuel additives cannot fix mechanical or electronic faults. They will not resolve:
Relying on additives instead of proper repairs can lead to repeated MOT failures.
Petrol additives mainly focus on improving combustion and reducing CO and HC emissions. Diesel additives often target soot reduction and injector cleanliness.
Diesel vehicles used for short trips benefit most, as additives may help reduce smoke levels before the test. However, DPF-related failures usually require professional attention.
To improve your chances of passing:
Fuel additives should be seen as a support tool, not a substitute for maintenance.
Fuel additives can improve MOT emission results in certain situations, especially where minor carbon build-up is the issue. However, they are not a guaranteed solution and will not fix serious mechanical faults. For best results, combine additives with proper servicing and informed preparation.
Do fuel additives guarantee an MOT emissions pass?
No. They may help marginal cases but cannot fix faulty components.
How long before the MOT should I use a fuel additive?
Ideally one to two weeks before, allowing time for cleaning and driving.
Are fuel additives safe for all vehicles?
Most are safe, but always check compatibility with your engine type.
Can additives damage engines or sensors?
Low-quality or excessive use may cause issues. Use reputable brands only.
Will additives help with a blocked DPF?
They may reduce soot levels but cannot clear severe blockages.
Can I check past emissions failures online?
Yes, you can review emissions-related MOT history using https://checkmot.com/.