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
How emissions defects are graded
Every MOT defect is recorded as dangerous, major or minor. Dangerous and major both fail the test; the difference is that a dangerous defect means the car is not roadworthy and must not be driven away. A minor defect passes but is recorded on the certificate, and an advisory is a note that something is wearing and will need attention before the next test. Tester notes in the emissions section use standard DVSA wording, which is why the phrasing on your certificate looks nothing like plain English — the pages above translate it.
Getting a emissions failure put right
You are free to take the car anywhere for the repair — there is no obligation to have the testing station do the work, and getting a second opinion on a big quote is sensible. Ask for the failed item to be shown to you on the ramp; a genuine defect is always something a mechanic can point at. Once repaired, a partial retest within ten working days covers the listed items without paying a second full test fee.
Avoiding it next year
Most emissions failures are found on a pre-MOT check for a fraction of the cost of a failure plus retest, because they are inspected before the clock is running. An MOT at Bob's Mechanical Repairs in Birnam, Dunkeld is £54.85, and anything found is explained and priced before work is agreed. Bob has 32 years on the tools, so the advice you get is what he would do with his own car.
Common questions
Can I drive away after a emissions MOT failure? Only if the old certificate is still valid and the defect is not dangerous. A defect recorded as dangerous means the car must not be driven on the road until it is repaired, and driving it can mean a fine and points.
Do I pay again for the retest? If the car is left at the testing station and the repair is done there, the partial retest is usually free. If you take it away and bring it back within ten working days, a partial retest is free for the listed items; after that a full test fee applies.
What does an advisory in this section mean? An advisory is a pass with a warning: the item is not bad enough to fail today but is wearing. Advisories on the same item two years running are the ones worth acting on before they become failures.
Decode your exact MOT wording or browse all MOT failure categories.