MOT defect file
Emissions above the permitted limit
Last updated
Quick answer
The vehicle exceeded its emissions threshold. Causes range from a due service to a failing catalyst or sensor.
- Typical cost
- £80–£900
- DIY viability
- Garage job
What you might notice
- Visible smoke from the tailpipe — blue for burnt oil, black for over-fuelling, white for coolant or unburnt diesel
- An engine management light, although plenty of emissions failures occur with no warning light showing
- A rougher idle, hesitation on part throttle, or fuel economy that has quietly slipped over a few months
- A sulphurous, rotten-egg smell, which points at the catalytic converter
What causes it
- A lazy or failed lambda sensor telling the engine to run rich, which is among the most common causes on a petrol car
- A catalytic converter that has aged out, been contaminated by oil or coolant, or broken up internally
- A diesel particulate filter loaded with soot because the car only does short journeys and never completes a regeneration
- Plain neglect: a clogged air filter, an overdue service, the wrong grade of oil, or worn spark plugs
Advisory or failure?
Emissions are pass or fail, with no advisory and no dangerous grading, so exceeding a limit is always a major defect. A catalyst-equipped petrol car is normally tested at fast idle against 0.3% carbon monoxide and 200 parts per million hydrocarbons, with lambda between 0.97 and 1.03, plus 0.5% carbon monoxide at natural idle — the manufacturer's own figures are used where they are tighter. Diesels are tested for smoke opacity: 1.5 m⁻¹ where a particulate filter was fitted as standard, 2.5 m⁻¹ for a non-turbocharged engine without one and 3.0 m⁻¹ for a turbocharged one, with lower plated values applied where the manufacturer specifies them. Two diesel rules catch people out: a vehicle with a factory-fitted DPF fails on any visible smoke of any colour whatever the meter reads, and if the tester finds evidence that the DPF has been removed they must refuse to test the vehicle at all.
Can you fix it yourself?
Garage job
Do the cheap things first, because they resolve a surprising number of marginal failures: a proper service with the correct oil, a new air filter, fresh plugs on a petrol engine, and a long run at motorway speed immediately before the test so the catalyst or DPF is fully up to temperature. Past that you need a gas analyser or a smoke meter to know whether anything you did actually changed the numbers, and fault codes alone will not tell you whether a lambda sensor is lazy or the catalyst behind it is spent. Treat additives sold as a guaranteed pass with scepticism — they occasionally nudge a borderline diesel and do nothing at all for a mechanical fault.
What it costs
The bottom of the band is a service with an air filter and plugs at £80 to £200, which clears a good share of marginal failures on its own. A lambda sensor is £40 to £120 in parts plus half an hour, assuming it unscrews without shearing. A forced regeneration or an off-car DPF clean is £150 to £400. The top of the band is a replacement catalytic converter or particulate filter — aftermarket cats start around £200 fitted, while an original-equipment DPF for a modern diesel can exceed £900 by itself. Most garages charge diagnostic time separately at £50 to £100, and it is money well spent before anything is replaced on a guess.
What happens if you ignore it
There is no ignoring it, because it is a failure and the car cannot legally be used on the road once the certificate expires. The underlying fault also tends to worsen itself: an engine running rich washes oil off the bores and poisons the catalyst, and a DPF allowed to load past a certain point can no longer be regenerated by driving and has to come off for cleaning or be replaced. Removing a DPF is not a way out — using a vehicle on the road with it taken out is an offence, and the MOT simply will not be carried out.
This is general guidance, not a diagnosis. The severity DVSA records depends on the specific fault found, so the same item can appear as an advisory on one test and a failure on another. Ask a qualified tester or mechanic to inspect the vehicle.
Has this been recorded on your car?
Check the full MOT history for any UK registration.