Smoke from the exhaust: what white, blue and black smoke mean
The colour of exhaust smoke is one of the most useful clues a car gives you. Thin white vapour on a cold morning is just condensation. Thick white smoke that smells sweet is coolant being burnt — stop driving. Blue smoke is oil, and black smoke is too much fuel for the air the engine is getting.
Pick the colour below for the likely causes, ranked, and what to do next.
Free interactive diagnosis
Is it petrol or diesel?
A hybrid counts as petrol. The likely causes are completely different between the two.
Is it safe to keep driving?
For a typical car with each symptom. These are general guidance, not a diagnosis — if you're unsure, don't drive it.
- Thin white smoke on a cold startUsually OK to drive
- Thick white smoke that won't clearDon't drive
- Blue or grey smoke from the exhaustDrive carefully
- Black smoke from the exhaustDrive carefully
- Smoke from under the bonnetDon't drive
Thin white smoke on a cold start
Usually fine to drive — get it checked when convenient
Usually a nuisance rather than a danger, and unlikely to strand you or cause other damage in the meantime. Get it checked when it's convenient — sooner if it gets worse or anything else changes.
Likely causes
- Normal condensation on a cold startMost likely
Thin white vapour that clears once the engine is warm. Nothing to fix.
If it's this: not urgent.
- Glow plugs or glow plug relayLikely
Hard to start when cold, fine once warm, and it gets worse week by week. A garage can test each plug in about ten minutes.
If it's this: get it checked soon.
What to do next
Almost certainly condensation, which is normal and disappears once warm. If it smells sweet or doesn't clear, see the thick white option.
Thick white smoke that won't clear
Stop — don't drive it
If you're driving, pull over when it's safe and switch off. Don't drive it again until it's been checked — have it recovered if you need to. Carrying on risks the engine, your brakes or your steering, and recovery costs less than the damage.
Likely causes
- Head gasketMost likely
Thick white sweet-smelling smoke, coolant disappearing with no visible leak, and bubbles in the expansion tank with the engine running.
If it's this: don't drive.
- Coolant leak (hose, radiator or water pump)Likely
The coolant is below MIN when the engine is cold, there's a sweet smell, and a puddle under the front of the car.
If it's this: get it checked soon.
- Fuel injectorLikely
A misfire or knock on one cylinder, and sometimes smoke. On a diesel a garage can do a leak-off test to find the bad one.
If it's this: get it checked soon.
What to do next
Check the coolant level when cold. Losing coolant with thick white smoke means it's being burnt — stop driving it before the engine overheats.
Blue or grey smoke from the exhaust
Drive carefully — book it in
You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.
Likely causes
- Engine burning oil (valve seals or piston rings)Most likely
Blue smoke on start-up or when you lift off the throttle, and topping up the oil between services.
If it's this: get it checked soon.
- Turbocharger or a leaking boost pipeLikely
A whistle or hiss under acceleration, loss of power, and sometimes blue or black smoke.
If it's this: get it checked soon.
- Engine or gearbox oil leakPossible
Dark brown drips under the engine, an oily underside, and a burning smell if it's dripping onto the exhaust.
If it's this: get it checked soon.
What to do next
It's burning oil. Check and top up the oil level now, and keep checking it — running low is how this becomes an engine failure.
Black smoke from the exhaust
Drive carefully — book it in
You can drive gently, but get it looked at within days rather than weeks. Left alone it will get worse, less safe or more expensive.
Likely causes
- EGR valve stickingMost likely
A rough idle, black smoke and a P0400-series code. Common on diesels that do mostly short, slow journeys.
If it's this: get it checked soon.
- Air flow sensor or a split boost hoseLikely
A flat spot or limp mode, codes P0100–P0104 or a boost fault. A split intake hose often hisses under load.
If it's this: get it checked soon.
- Fuel injectorLikely
A misfire or knock on one cylinder, and sometimes smoke. On a diesel a garage can do a leak-off test to find the bad one.
If it's this: get it checked soon.
- Turbocharger or a leaking boost pipePossible
A whistle or hiss under acceleration, loss of power, and sometimes blue or black smoke.
If it's this: get it checked soon.
What to do next
A single black puff from a diesel under hard acceleration or during a DPF regeneration is normal. Constant black smoke means it's over-fuelling — have the codes read.
Smoke from under the bonnet
Stop — don't drive it
If you're driving, pull over when it's safe and switch off. Don't drive it again until it's been checked — have it recovered if you need to. Carrying on risks the engine, your brakes or your steering, and recovery costs less than the damage.
Likely causes
- Engine or gearbox oil leakMost likely
Dark brown drips under the engine, an oily underside, and a burning smell if it's dripping onto the exhaust.
If it's this: get it checked soon.
- Coolant leak (hose, radiator or water pump)Likely
The coolant is below MIN when the engine is cold, there's a sweet smell, and a puddle under the front of the car.
If it's this: get it checked soon.
- Exhaust blowing, corroded or loosePossible
A rattle or blowing noise from underneath, louder on a cold start. If you can smell exhaust fumes inside the car, stop driving it.
If it's this: get it checked soon.
What to do next
Pull over, switch off and get everyone out. Don't open the bonnet if you can see flames. Steam that smells sweet is coolant; blue smoke that smells acrid is oil on the exhaust.
A shortlist, not a diagnosis
The ranking is the one a mechanic would give over the counter for a typical car — nobody can road-test a car over the internet, and the real cause may not be listed. Add the registration in the checker above to have it checked against that car's MOT advisories, mileage and engine. Have the car checked by a qualified mechanic before relying on it, and if you're unsure whether it's safe, don't drive it.
Common questions
Is white smoke from the exhaust bad?
Not if it's thin, clears once the engine warms up and has no smell — that's condensation. Thick white smoke that keeps coming, smells sweet, or comes with a falling coolant level is coolant being burnt, often a head gasket, and you should stop driving to avoid wrecking the engine.
What does blue smoke from the exhaust mean?
Oil is getting into the engine's combustion chambers — usually worn valve stem seals or piston rings, or on a turbocharged car a leaking turbo seal. Check the oil level often: running low on oil does far more damage than the smoke itself.
Why is my car blowing black smoke?
Too much fuel for the air available. A sticking EGR valve, a split boost hose, a faulty air flow sensor or a leaking injector are the usual causes. A puff under hard acceleration can be normal on an older diesel; constant black smoke isn't, and on a newer diesel it loads up the particulate filter.
Related symptoms
Engine management light
Engine management light on: causes and is it safe to drive?
Running poorly
Car running badly? Losing power, misfiring, stalling or juddering
Leaks
Car leaking fluid? Identify it by colour, smell and position
All symptoms: Engine management light · Other warning lights · Won't start · Running poorly · Leaks · Brakes · Steering & vibration · Noises · Symptom checker