When preparing for your MOT test, most drivers focus on visible issues like tyres, lights, and brakes. However, what many overlook is the quality of fuel being used — a factor that can have a surprising impact on your MOT emissions results. Poor fuel quality doesn’t just harm your vehicle’s performance; it can also lead to an emissions failure during your MOT.

If your MOT due date is approaching, you can easily check it at Check MOT.

How Fuel Quality Affects Vehicle Emissions

Fuel acts as the lifeblood of your engine, and its quality directly influences combustion efficiency. When you use low-grade or contaminated fuel, incomplete combustion occurs, leading to:

• Higher levels of carbon monoxide (CO) and hydrocarbons (HC) in the exhaust
• Soot build-up in diesel particulate filters (DPF)
• Increased engine deposits, which affect sensors and catalytic converters

These issues make it harder for your car to maintain optimal air-fuel balance — something that’s measured closely during the MOT emissions test.

Diesel vs Petrol: The Impact of Poor Fuel Quality

The effects of poor fuel quality differ between petrol and diesel vehicles:

• Petrol engines: Poor fuel can cause incomplete combustion, fouled spark plugs, and higher HC emissions. It may also damage the oxygen sensor, which regulates the fuel-air mixture.
• Diesel engines: Low-quality diesel or water-contaminated fuel can clog injectors and cause excessive smoke, high particulate emissions, or even DPF failure — a common reason for MOT failure among diesel cars.

Drivers using cheap or unbranded fuel stations more frequently may experience these issues sooner than those sticking to reputable providers.

How to Avoid MOT Emissions Failures Caused by Poor Fuel

Here are a few tips to reduce your risk of failing your emissions test due to poor fuel quality:

1. Use reputable fuel brands – Branded fuels contain detergents and additives that help keep your engine clean.
2. Add fuel system cleaners – Periodic use of an approved additive can reduce carbon build-up and restore efficiency.
3. Avoid running on empty – Low fuel levels can cause sediment from the bottom of the tank to circulate, affecting filters and injectors.
4. Service your vehicle regularly – Routine maintenance, including oil changes and filter replacements, supports cleaner emissions.
5. Take your car for a motorway run before the MOT – A longer, high-speed drive can help burn off carbon deposits in the exhaust system, especially for diesel vehicles.

What If You Fail the MOT Emissions Test?

If your car fails the emissions test, it’s not the end of the road. Many garages offer MOT retests after repairs. Often, the issue can be resolved through a fuel system clean, filter replacement, or software update.

You can find local MOT centres and check your vehicle’s history easily on Check MOT.

FAQs

1. Can bad fuel really cause an MOT failure?

Yes. Poor-quality or contaminated fuel can increase harmful emissions and cause your car to fail the emissions portion of the MOT test.

2. Should I use premium fuel before an MOT?

Using premium or high-quality fuel before your MOT may help clean the engine and reduce emissions slightly, though it’s not a guaranteed fix.

3. Can a fuel additive help pass the MOT?

Additives can clean the fuel system and improve combustion, potentially lowering emissions, but they should be used as part of regular maintenance.

4. What’s the best way to prepare for the emissions test?

Keep your engine serviced, use quality fuel, drive the car for at least 20 minutes before the test, and ensure the exhaust system is in good condition.

Using high-quality fuel not only improves performance but also increases your chances of passing the MOT emissions test on the first try. For more MOT guidance and to check your MOT status, visit Check MOT.