Why Does the MOT Include an Exhaust Emissions Test?

Why the MOT Tests Emissions

The MOT includes an emissions test to confirm that your car meets the environmental standards set in law. It measures the harmful gases or smoke leaving the exhaust and compares them against the limits for your car's age and fuel type. A car that pollutes more than allowed fails, since this is treated as a serious environmental issue.

The test is about air quality as much as the mechanical health of the car. A well running engine burns fuel cleanly, so high emissions often point to a fault that also wastes fuel. In that sense passing the emissions test tends to go hand in hand with a car that is running efficiently. Our overview on What Is an MOT explains where emissions sit within the full test.

Exactly what is measured depends on whether the car runs on petrol or diesel, since the two produce different pollutants. The rest of this guide breaks down each, along with the common reasons a car fails and what electric cars mean for the test.

What Is Measured on Petrol and Diesel

The emissions test differs by fuel, because petrol and diesel engines produce different byproducts. The table below sets out what each is checked for.

For a petrol car the tester reads the gases at the tailpipe and checks the air to fuel ratio. For a diesel the focus is on smoke, with a modern diesel expected to be very clean. Our guide on Electric Cars and MOT explains how electric cars are tested without an emissions check.

Common Reasons Cars Fail on Emissions

Most emissions failures come down to a fault that stops the engine burning fuel cleanly. Knowing the common causes helps you spot a problem before the test.

Many of these faults also show as poor running or higher fuel use, so an emissions problem is often a sign the car needs attention anyway. Our guide on Top Reasons Cars Fail MOT covers the other top failure areas.

How to Give Emissions the Best Chance

A little care in the run up to a test can help a borderline car through. Making sure the engine is warm and running well matters, since a cold or poorly serviced engine produces more emissions. A good long drive before the test helps a diesel filter clean itself and clears the system.

Keeping up with servicing is the real answer, as fresh oil, a clean air filter and healthy spark plugs all keep emissions low. If a warning light linked to the engine is showing, it is worth sorting before the test, since it can point to an emissions related fault. Our guide on Warning Lights on MOT explains which warning lights cause a fail.

Why Diesels and the DPF Deserve Care

Diesel cars deserve a special mention, because the diesel particulate filter is a common source of trouble at test time. This filter traps soot from the exhaust and burns it off during longer, faster drives. A car used only for short local trips may never get hot enough to clean the filter, so soot builds up and the car can produce visible smoke.

A blocked filter or one that has been removed entirely is a serious emissions failure. Removing a filter is itself against the rules. The best prevention is a regular longer drive at speed to let the filter regenerate, along with keeping the car serviced. If you mostly do short journeys, an occasional run on a faster road does the diesel a real favour. Our guide on Top Reasons Cars Fail MOT covers the other areas cars commonly fail on.

Keep Emissions in Check

This guide is part of our full MOT Guides hub, where the testing, cost and rules articles sit together so you can move to the exact detail you need next. When it is time to book, our team offers full MOT testing and repairs from our centre in Ely. You may also find it useful to read What an MOT Checks, Electric Cars and MOT and Top Reasons Cars Fail MOT.

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