Diesel smoke over the limit
How it can appear on an MOT certificate
- “Diesel smoke opacity exceeds limits”
- “Exhaust emits dense blue or clearly visible black smoke”
What it is
For diesel cars, the MOT measures how much the exhaust smoke blocks light (opacity). Visible blue or black smoke is also checked.
Why it matters
Heavy smoke can point to problems with injectors, the turbo, the air supply or the diesel particulate filter (DPF).
What to check when viewing a car
Watch the exhaust when the engine is revved and on a test drive under acceleration. Ask about DPF and injector history on higher-mileage diesels.
Has the car you're looking at had this before?
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What the MOT categories mean
- Dangerous
- A direct and immediate risk to road safety, or a serious impact on the environment. The car fails and shouldn't be driven until it's repaired.
- Major
- It may affect the car's safety, put other road users at risk or harm the environment. The car fails and should be repaired straight away.
- Minor
- No significant effect on safety or the environment. The car passes, but it should be repaired as soon as possible.
- Advisory
- Something to keep an eye on that could need repairing in future. The car passes.
Related: exhaust and emissions
- Exhaust leak
- Exhaust insecure or mountings damaged
- Emissions over the limit
- Engine warning light (MIL) on
- Catalytic converter missing or not working
Buying a used car?
General information about this type of MOT defect, not advice about a particular car. How serious it is depends on the car; have any car you're buying inspected by a qualified mechanic.