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.
Wording as it commonly appears on an MOT certificate: “Exhaust emissions exceed the default or manufacturer's limit”.
What does it mean?
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.
How serious is it?
Usually recorded as: Major. The tester assesses the actual vehicle on the day, so your certificate is what counts.
What normally causes it?
- A tired or contaminated lambda sensor giving the ECU bad information
- A catalytic converter past its best, common on high-mileage cars
- Ignition faults — worn plugs, weak coils — causing incomplete combustion
- An air leak after the mass airflow sensor leaning the mixture out
- Simply not being properly warmed up before the test
- Something as basic as a long-overdue service with a clogged air filter
What a mechanic would check
Plug in and read live data first — fuel trims and lambda sensor activity tell you far more than the printout does. Short-term and long-term trims well into positive numbers point at an air leak or a fuel delivery problem; a lazy front sensor points at the sensor itself. I'd never fit a catalytic converter on the strength of a failed gas test alone.
What might need replacing
- A service — plugs, air filter, oil — which fixes a fair share of marginal failures
- A lambda sensor
- An intake leak repaired
- A catalytic converter, but only once everything upstream has been proven good
Can you still drive it?
It's a failure item rather than an immediate safety issue. The car may drive perfectly normally, but the emissions won't fix themselves and a retest costs money if you go back without addressing the cause.
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This is general guidance from a UK mechanic, not a substitute for a physical inspection. Only the tester who inspected your vehicle can confirm what was recorded and why.