What is the MOT smoke limit for a diesel?

Where the manufacturer's plate shows an opacity value, that value is used with 0.05 added for tolerance. Otherwise the default depends on the date of first use, not on what is fitted: 0.7 m⁻¹ from 1 January 2014, 1.5 m⁻¹ between 1 July 2008 and 31 December 2013, and before that 2.5 m⁻¹ without a turbocharger or 3.0 m⁻¹ with one. Exceeding the limit is a major defect.

When is exhaust smoke a dangerous MOT defect?

Where the exhaust emits smoke or vapour of any colour in enough quantity to obscure the vision of other road users, the grading is dangerous rather than major. That means the car should not be driven until it is fixed, and using it in that condition is an offence in its own right, separate from the failed test.

What does black, blue or white smoke from a diesel mean?

Black smoke under acceleration usually means over-fuelling from a worn or leaking injector, or an air supply problem such as a blocked filter or a stuck exhaust gas recirculation valve. Blue haze at idle or on the overrun is oil reaching the combustion chamber, often past turbocharger seals. White smoke that does not clear as the engine warms is different again from ordinary cold-morning steam.

MOT defect file

Diesel emitting visible smoke

Last updated 27 July 2026

Quick answer

Smoke was visible from the tailpipe during the test. On any diesel built with a particulate filter, smoke of any colour at all is a failure regardless of what the opacity meter reads.

Typical cost: £150–£1,200
DIY viability: Garage job

Smoke is measured during free acceleration and the manufacturer’s plate limit takes precedence where shown. In the fallback categories illustrated, lower bars mean stricter limits. Any visible smoke from a DPF-equipped vehicle is itself a failure.

What you might notice

A puff of black smoke when you accelerate hard or pull away uphill
Blue haze at idle or on the overrun, which points at oil getting into the combustion chamber
White smoke that does not clear as the engine warms, as distinct from ordinary cold-morning steam
A car that has quietly started using oil, or that has needed a regeneration message clearing more often than it used to

What causes it

A worn or leaking injector over-fuelling one cylinder, which is the usual answer for black smoke on a modern diesel
Turbocharger seals or bore wear letting oil into the intake, which produces blue smoke and rising oil consumption
A clogged intake, a stuck exhaust gas recirculation valve or a blocked air filter, all of which starve the engine of air
A particulate filter that is loaded, cracked or no longer sealing, so soot passes straight through it

Advisory or failure?

Diesel smoke is measured for opacity, and the limit depends on the car. Where the manufacturer's plate shows a value, that value is used, with 0.05 added to allow for manufacturing tolerance. Where there is no plated value the defaults apply: 2.5 m⁻¹ for a non-turbocharged engine and 3.0 m⁻¹ for a turbocharged one on cars first used before 1 July 2008, 1.5 m⁻¹ for cars first used between 1 July 2008 and 31 December 2013, and 0.7 m⁻¹ for anything first used on or after 1 January 2014. Exceeding either the plated or the default limit is a major defect. Two rules sit alongside the meter and are the ones that catch owners out. On a car fitted with a particulate filter, the tester also checks that no visible smoke of any colour comes out during the metered test, and any visible smoke at all is a major defect regardless of the reading. And where the exhaust emits excessive smoke or vapour of any colour to an extent likely to obscure the vision of other road users, the grading is dangerous. Petrol cars are assessed visually rather than by meter, where dense blue or clearly visible black smoke for five continuous seconds at idle is a major defect.

Can you fix it yourself?

Garage job

Do the cheap and honest things first, because they clear a real share of marginal failures: an overdue service with the correct oil, a new air filter, and a sustained motorway run immediately before the test so the engine and the filter are properly hot rather than half-warm from a five-minute drive. Past that you are guessing without a smoke meter, and injector, turbo and filter faults all present as smoke. Be wary of additives sold as a guaranteed pass — they occasionally help a borderline car and do nothing at all for a mechanical fault. One safety point that is worth knowing before you book: a tester may refuse the smoke test altogether if the engine is low on oil, sounds abnormal, has low oil pressure or has a cambelt in poor condition, because the test itself involves holding the engine at full fuel.

What it costs

The bottom of the band is a service, an air filter and a decent run at £150 to £250, which resolves more marginal cases than people expect. A stuck exhaust gas recirculation valve is £120 to £350 fitted. A single injector is £150 to £450 depending on the engine, and injectors seized into the head are the most common source of unplanned labour — a snapped one can add several hours. A forced regeneration is £80 to £150; an off-car filter clean £150 to £400. The top of the band is a turbocharger or a replacement particulate filter, both of which can exceed it on their own. Budget £50 to £100 for diagnosis with a smoke meter before authorising parts, because guessing here is expensive.

What happens if you ignore it

Smoke is unburnt fuel or burnt oil, so the fault behind it is consuming something it should not and getting worse while you drive. Over-fuelling washes the bores, dilutes the oil and loads the particulate filter faster than it can regenerate, which turns an injector job into a filter job. Blue smoke from a failing turbo can end with the turbo shedding into the engine. There is also the road-legal position: a vehicle emitting avoidable smoke can be dealt with separately from the MOT, and where the smoke is thick enough to obscure other drivers' view the test grades it dangerous, which means the car should not be driven until it is fixed.

Common questions

What is the MOT smoke limit for a diesel?: Where the manufacturer's plate shows an opacity value, that value is used with 0.05 added for tolerance. Otherwise the default depends on the date of first use, not on what is fitted: 0.7 m⁻¹ from 1 January 2014, 1.5 m⁻¹ between 1 July 2008 and 31 December 2013, and before that 2.5 m⁻¹ without a turbocharger or 3.0 m⁻¹ with one. Exceeding the limit is a major defect.
Does any visible smoke fail the MOT on a diesel with a DPF?: Yes. On a car built with a particulate filter the tester also checks that no visible smoke of any colour leaves the tailpipe during the metered test, and any visible smoke at all is a major defect regardless of what the meter reads. Coming in under the opacity limit does not save a car that is visibly smoking.
When is exhaust smoke a dangerous MOT defect?: Where the exhaust emits smoke or vapour of any colour in enough quantity to obscure the vision of other road users, the grading is dangerous rather than major. That means the car should not be driven until it is fixed, and using it in that condition is an offence in its own right, separate from the failed test.
What does black, blue or white smoke from a diesel mean?: Black smoke under acceleration usually means over-fuelling from a worn or leaking injector, or an air supply problem such as a blocked filter or a stuck exhaust gas recirculation valve. Blue haze at idle or on the overrun is oil reaching the combustion chamber, often past turbocharger seals. White smoke that does not clear as the engine warms is different again from ordinary cold-morning steam.
How much does it cost to fix a diesel that failed on smoke?: A service with the correct oil, an air filter and a sustained motorway run immediately before the retest is £150 to £250 and clears more marginal cases than people expect. A stuck exhaust gas recirculation valve is £120 to £350 fitted, a single injector £150 to £450, a forced regeneration £80 to £150 and an off-car filter clean £150 to £400. A turbocharger or a replacement filter is what reaches £1,200.

Warning lights behind this

This defect is what a tester writes down. What you see from the driver’s seat is a symbol, and it is worth checking which one, because manufacturers reuse the same shape for more than one system.

Fault codes behind this failure

A failure certificate records that the lamp was lit, not what lit it. If this is what your certificate says, one of these codes is usually stored behind it, and reading it is the difference between a diagnosis and a guess.

Look up any of the other 11, or search the full index by code or symptom.

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.