Exhaust gases or smoke above the limits for the car, an emissions test that can't be completed, or emission control equipment such as a catalytic converter or diesel particulate filter that is missing, obviously modified or obviously defective.
Testers grade each defect: a major or dangerous defect fails the MOT, a minor one passes but is written on the certificate.
| Defect | Result |
|---|---|
| Emission control equipment fitted by the manufacturer: missing, obviously modified or obviously defective | FailsMajor |
| An induction or exhaust leak that could affect emissions levels | FailsMajor |
| Defect | Result |
|---|---|
| Emissions levels exceed the manufacturer’s specified limits | FailsMajor |
| Emissions levels exceed default limits | FailsMajor |
| Lambda coefficient outside the default limits or the range specified by the manufacturer | FailsMajor |
| Emissions test unable to be completed | FailsMajor |
| Engine is idling clearly above its normal idle speed | FailsMajor |
| Exhaust emits dense blue or clearly visible black smoke for a continuous period of 5 seconds at idle | FailsMajor |
| Exhaust emits excessive dense blue or clearly visible black smoke during acceleration which would obscure the view of other road users | FailsMajor |
| Defect | Result |
|---|---|
| Emission control equipment fitted by the manufacturer missing, obviously modified or obviously defective | FailsMajor |
| An induction or exhaust leak that could affect emissions levels | FailsMajor |
| Evidence that the diesel particulate filter has been tampered with | FailsMajor |
| Defect | Result |
|---|---|
| Smoke opacity levels exceed the manufacturer’s specified limit | FailsMajor |
| Smoke opacity levels exceed default limit | FailsMajor |
| Exhaust emits excessive smoke or vapour of any colour to an extent likely to obscure the vision of other road users | FailsDangerous |
| Exhaust on a vehicle fitted with a diesel particulate filter emits visible smoke of any colour | FailsMajor |
| Emissions test unable to be completed | FailsMajor |
| Emissions test not completed because smoke levels are significantly in excess of the specified limit values | FailsMajor |
From DVSA's MOT inspection manual for cars and passenger vehicles. Contains public sector information licensed under the Open Government Licence v3.0.
In 2025, 375,008 car MOTs failed on emissions: 1.19%, or about 1 in 84. Testers also noted it as an advisory on 22,560 MOTs: something to keep an eye on that didn't fail the test.
Share of the MOTs that failed on emissions. One MOT can fail for more than one reason.
Share of car MOTs in each mileage band that failed on emissions.
3.23% of MOTs on cars registered in 2005 failed on emissions, against 0.06% for cars registered in 2022.
Models that failed on emissions at least 1.15 times as often as the average car of the same age and mileage, out of 94 with at least 30 MOTs failed on it.
Least often: Volvo XC60 (0.1×), VW Tiguan (0.1×), Audi Q5 (0.2×), Mercedes GLA (0.2×), Volvo XC90 (0.2×).
Exhaust gases or smoke over the limit
In 2025, 375,008 car MOTs failed on emissions, about 1 in 84.
1 in 84car MOTs failed on emissions in 2025
Exhaust gases or smoke above the limits for the car, an emissions test that can't be completed, or emission control equipment such as a catalytic converter or diesel particulate filter that is missing, obviously modified or obviously defective.
In 2025, 375,008 car MOTs failed on emissions: 1.19%, or about 1 in 84.
Against the average car of the same age and mileage, the Peugeot 208 (5.4 times as often), MG ZS (4.3 times as often) and Peugeot 2008 (3.5 times as often), going by 2025 MOT results.