Emissions is the category where the certificate gives you a number but no diagnosis at all. It tells you the reading was outside the limit; it doesn't tell you which sensor, injector or filter caused it.
Emissions is the category where the certificate gives you a number but no diagnosis at all. It tells you the reading was outside the limit; it doesn't tell you which sensor, injector or filter caused it.
It's also the one where the fix ranges from almost nothing to seriously expensive. A diesel that fails on smoke because it's spent six months doing short cold trips is a different problem from one with a failed EGR cooler, even though the certificate wording can be identical.
Below are the emissions entries I see most, and the sensible order to investigate them in before spending money.
Exhaust emissions exceed the default or manufacturer's limit
The gas analyser recorded readings outside the permitted limits for your engine. On a petrol car that usually means the CO, HC or lambda figures — those numbers describe how well the engine is burning fuel and how well the catalyst is cleaning up what's left.
Full explanation of Exhaust emissions exceed the default or manufacturer's limit
Engine malfunction indicator lamp indicates a fault
The engine management light was on with the engine running. It's a straight failure regardless of how the car drives — the MOT doesn't care what the fault is, only that the system is reporting one.
Full explanation of Engine malfunction indicator lamp indicates a fault
Smoke density exceeds the limit for this vehicle
During the diesel smoke test the opacity reading was above the limit for your vehicle. In plain terms, too much soot is coming out under a free acceleration test.
Full explanation of Smoke density exceeds the limit for this vehicle
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