Why do MOT tests include an exhaust emission test?

Friday 16th December 2022

Signs you need a new exhaust

What is an exhaust emission test? 

An exhaust emissions test determines the level of air pollutants emitted from the exhaust of your vehicle. An emissions test aims to help reduce the number of pollutants that are harmful to the environment. Although diesel and petrol vehicle emissions tests are different, both are tested as part of an MOT and usually tested using a meter. When the emissions test is complete, you will be aware of how much pollution your vehicle is producing.  


An emission test for a petrol car measures the gasses that exit the exhaust. The required reading must be within specified limits of the vehicle. 


A diesel emission test measures ‘opacity’ which refers to the density of particles that exit the exhaust. Light is used to measure opacity, which an emission analyser uses the data to calculate an opacity reading. The ‘plate value’ of a car is used during a diesel emission test. This holds the manufacturer’s data for the level at which the car must be below in order to pass an emission test.  

Why do MOT tests include an exhaust emission test? 

As there is more focus on reducing pollution levels that are produced by cars, especially those with diesel engines. Some limits had to be introduced to ensure that cars aren’t producing excessive emissions that are harmful to the planet. Emission tests are carried out to ensure that your vehicle’s engine is operating efficiently, so that the pollution produced by the engine is kept to a minimum.  

How to reduce your cars emissions 

As cars are getting more efficient and cleaner year after year, it is essential your car is in good working order. This means your emissions should be in line with what the manufacturer specifies. If your vehicle is relatively new, you are unlikely to fail your MOT on emission levels, however, if you’re driving an older car that may be diesel, then you could fail an MOT based on high emission levels. There are ways to reduce your car emissions, these include: 

Servicing your car regularly  

Regular servicing helps improve your car’s emissions as it involves your engine oil being changed. Oil lubricates and cleans the engine and when it is changed, it will help to keep the engine running effectively. During a service, air filters will also be changed which can also lower your car emissions. A clogged-up air filter can impact the engine, this will lead to a build-up in the engine which can cause damage.  

Change your fuel 

Switching up your fuel to ‘premium’ can help reduce your emissions as it is better quality or grade of fuel. They also contain cleaning chemicals which will remove dirt and any build-up in your engine.  

Check your tyre pressure 

Ensuring your tyres are set at the correct pressure can help reduce your emissions. If your tyres are under-inflated, not only is your car more dangerous to drive, but it may also lead to increased fuel usage and higher emissions.  

Keep on top of your emissions by saving money and the environment with our fuel saver service as well as booking in for regular service. Simply visit your local Formula One Autocentre or for more information, contact our helpful team today who will be happy to help. 

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