What Happens During an MOT Emissions Test? A Behind-the-Scenes Look

When your car goes in for its MOT, one of the most important checks is the emissions test. This test measures the amount of harmful gases your vehicle produces to ensure it meets UK environmental standards. If your car fails this part of the MOT, it can’t legally be driven on the road until the issue is fixed.

In this guide, we’ll take a behind-the-scenes look at what happens during an MOT emissions test and how you can prepare your car to pass first time.

Why the Emissions Test Matters

The MOT emissions test is designed to help reduce air pollution and protect public health. Vehicles that emit excessive levels of carbon monoxide (CO), hydrocarbons (HC), or nitrogen oxides (NOx) contribute to smog, respiratory illnesses, and climate change.

Passing the emissions test isn’t just about legal compliance—it’s also about doing your part for the environment.

How the MOT Emissions Test Is Carried Out

The process depends on whether your car has a petrol or diesel engine.

Petrol Vehicles

For petrol cars, the tester will:

1. Warm up the engine – This ensures more accurate results.
2. Insert a probe into the exhaust – This measures gases like CO and HC.
3. Run the engine at different speeds – The tester checks emissions while idling and at higher revs.
4. Compare results with legal limits – These limits vary depending on your vehicle’s age and model.

Diesel Vehicles

For diesel cars, the process focuses on smoke opacity rather than direct gas measurement:

1. Revving the engine – The tester accelerates the engine sharply to measure smoke density.
2. Smoke meter test – This device calculates the opacity level to ensure it’s within legal limits.
3. Visual inspection – The tester checks for excessive smoke coming from the exhaust.

Common Reasons for Emissions Test Failure

Some of the most frequent causes include:

• Faulty oxygen sensors
• Worn or damaged catalytic converter
• Dirty air filter
• Poor engine maintenance
• Excessive oil consumption

For diesels, a blocked Diesel Particulate Filter (DPF) is a common reason for failure.

How to Improve Your Chances of Passing

Here are a few tips to prepare for your MOT emissions test:

• Service your car regularly – Replace spark plugs, filters, and fluids as recommended.
• Use high-quality fuel – This can help reduce carbon build-up.
• Take your car for a long drive before the MOT – Known as an “Italian tune-up,” this helps burn off deposits.
• Check for warning lights – If your dashboard shows an engine or DPF warning, fix it before the test.

Final Thoughts

Understanding what happens during an MOT emissions test can help you take the right steps to keep your car roadworthy and environmentally friendly. A well-maintained engine not only passes the MOT more easily but also runs more efficiently, saving you money on fuel in the long run.

You can check your MOT history and next due date instantly at CheckMOT.com.