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.
Repair cost
The range here is enormous, from a routine service to a catalytic converter. That's exactly why diagnosis first matters — replacing a cat that wasn't the problem is one of the most expensive mistakes in motoring.
Before the retest
Fix the cause, not the reading: a fault code, sensor or air leak that made the car fail will make it fail again. Make sure no engine warning light is lit. Drive the car properly on the way to the retest — at least 20 minutes at normal road speeds — so the engine and catalytic converter are fully up to temperature. Never remove or tamper with a diesel particulate filter; that is an automatic failure.
Retest rules
If the vehicle stays at the test centre for repair and is retested within 10 working days, a partial retest is often free; some items qualify for a free retest if you bring the car back by the end of the next working day. The exact conditions are set out on GOV.UK, so confirm with the test centre before you leave. A major or dangerous failure means the car cannot legally be driven away unless the previous MOT is still valid and nothing dangerous was recorded — check your certificate first.
Related help
More emissions failures · MOT failure decoder · Ask a mechanic about your certificate
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.