What is the MOT smoke limit for a diesel?

The limit comes from the manufacturer's plate where there is one, with 0.05 added to allow for manufacturing tolerance. Without a plated value the default depends on the date of first use: 0.7 m⁻¹ from 1 January 2014, 1.5 m⁻¹ between 1 July 2008 and 31 December 2013, and 2.5 m⁻¹ for an older non-turbocharged engine or 3.0 m⁻¹ for a turbocharged one.

Will any smoke from my diesel fail the MOT?

If the car left the factory with a diesel particulate filter, yes. Any visible smoke of any colour is a failure whatever the opacity meter reads, which catches out owners whose measured numbers would have passed. Smoke thick enough to obscure another driver's vision is the one emissions item graded dangerous.

Does a removed DPF fail the MOT?

Yes, as a major defect. Evidence that the diesel particulate filter has been removed or tampered with fails in its own right, and so does a missing filter where one was fitted from the factory. Removal is not a route to a pass, and using a vehicle on the road with the filter taken out is an offence in itself.

How do I pass an emissions retest?

Start with the cheap work, because it clears a good share of marginal failures: a proper service, a new air filter, fresh plugs on a petrol engine, and a long motorway run immediately before the test so the catalyst or filter is fully hot. That is £80 to £200. Beyond it, a lambda sensor is £40 to £120 in parts, a forced regeneration or off-car DPF clean £150 to £400, and a replacement catalyst or particulate filter more again.

MOT defect file

Emissions above the permitted limit

Last updated 8 September 2026

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

The measured value must not exceed the applicable CO or HC limit, and lambda must remain within its permitted band. Here CO and lambda pass, but HC exceeds 200 ppm and fails. CO and HC figures are illustrative; the tester uses the limit for the particular vehicle.

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, and the limit comes from the manufacturer's plate where there is one, with 0.05 added to allow for manufacturing tolerance. Where there is no plated value the default depends on when the vehicle was first used: 0.7 m⁻¹ from 1 January 2014, 1.5 m⁻¹ between 1 July 2008 and 31 December 2013, and 2.5 m⁻¹ for an older non-turbocharged engine or 3.0 m⁻¹ for a turbocharged one. 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 evidence that the DPF has been removed or tampered with is a major defect in its own right. Smoke thick enough to obscure another driver's vision is the one emissions item graded dangerous.

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 — a filter that is missing or has been tampered with is a major defect and an outright failure, and using the vehicle on the road in that state is a separate offence carrying a fine of up to £1,000 for a car and £2,500 for a light goods vehicle.

Common questions

What are the MOT emissions limits for a petrol car?: 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. Emissions are pass or fail with no advisory, so exceeding a limit is always a major defect.
What is the MOT smoke limit for a diesel?: The limit comes from the manufacturer's plate where there is one, with 0.05 added to allow for manufacturing tolerance. Without a plated value the default depends on the date of first use: 0.7 m⁻¹ from 1 January 2014, 1.5 m⁻¹ between 1 July 2008 and 31 December 2013, and 2.5 m⁻¹ for an older non-turbocharged engine or 3.0 m⁻¹ for a turbocharged one.
Will any smoke from my diesel fail the MOT?: If the car left the factory with a diesel particulate filter, yes. Any visible smoke of any colour is a failure whatever the opacity meter reads, which catches out owners whose measured numbers would have passed. Smoke thick enough to obscure another driver's vision is the one emissions item graded dangerous.
Does a removed DPF fail the MOT?: Yes, as a major defect. Evidence that the diesel particulate filter has been removed or tampered with fails in its own right, and so does a missing filter where one was fitted from the factory. Removal is not a route to a pass, and using a vehicle on the road with the filter taken out is an offence in itself.
How do I pass an emissions retest?: Start with the cheap work, because it clears a good share of marginal failures: a proper service, a new air filter, fresh plugs on a petrol engine, and a long motorway run immediately before the test so the catalyst or filter is fully hot. That is £80 to £200. Beyond it, a lambda sensor is £40 to £120 in parts, a forced regeneration or off-car DPF clean £150 to £400, and a replacement catalyst or particulate filter more again.

Cars this happens to most

Of every model with at least 2,000 tests in the DVSA data, these record emissions above the permitted limit on the largest share of their MOTs. Across all of them the typical rate is 1.1% of tests.

Peugeot 10618%
747 times in 4,178 tests
MG Mgf15%
773 times in 5,296 tests
Mazda RX-813%
332 times in 2,520 tests
Rover Mini12%
1,358 times in 10,919 tests
Citroen Saxo12%
354 times in 2,906 tests
Toyota MR210.0%
969 times in 9,713 tests
Mini Mini9.4%
23,206 times in 246,751 tests
Nissan 350 Z9.1%
435 times in 4,803 tests

A high rate here is a property of the fleet as much as the car: these models skew old, and almost everything fails more with age. It is a reason to check the item on a used example, not proof the model is badly built. Models with fewer than 2,000 tests are left out because a rate drawn from a few hundred cars moves too much to rank.

Pass rates for every make and model

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.