Checks for Exhaust Emissions


Most UK motorists will be aware that exhaust emissions are on the list of criteria checked during their annual MOT (Ministry of Transport) vehicle check. For the vehicle to pass the MOT, it must meet the minimum requirements for exhaust emissions. However, these requirements can vary according to the age of the vehicle and the type of fuel used by the vehicle.

To run the test on exhaust emissions, the car is connected to an analytical computer which can measure carbon dioxide, hydrocarbons and carbon monoxide levels emitted by the exhaust. If these exceed the recommended level for the car’s age and fuel type, the vehicle will fail the MOT.

At the time of writing, these are as follows:

Carbon Monoxide Emissions

For vehicles registered between 1st August 1975 and 31st July 1986, the CO level must not exceed 4.5% by volume.

For vehicles registered between 1st August 1986 and 31st July 1992, the CO level must not exceed 3.5% by volume.

Vehicles registered after 1st August 1992 must conform to the manufacturers specification (the MOT testing garage usually checks these values on a database). The carbon monoxide level is tested at two speeds; “idle” and “fast idle”. Although the specification varies between car models, the guidelines are generally 0.5% at idle and 0.3% at fast idle.

Hydrocarbon Emissions

For vehicles registered between 1st August 1975 and 31st July 1992, the HC emissions must not exceed 1200 ppm (parts per million).

Vehicles registered after 1st August 1992 must conform to the manufacturer’s specification. The hydrocarbon level is again tested at “fast idle” speed. Although the specification varies between car models, the guidelines are generally no more than 200 ppm at fast idle.

Diesel Cars

The only test carried out on diesel cars is smoke density (as related to particulate matter emissions). To carry out this test, the engine is accelerated several times and the vehicle must not emit “dense blue or clearly visible black smoke for a period of more than 5 seconds” at idle or “dense blue or clearly visible black smoke during acceleration which would obscure the view of other road users”.