MOT advisory explained
“Exhaust emits excessive smoke or vapour”
An advisory that can mean three different things depending on the colour of the smoke. Blue is oil, black is fuel, white is coolant or just cold weather. The colour decides whether this is a bottle of oil, a sensor, or a serious engine repair, and the data shows blue is the one to worry about.
The tester saw more smoke from the tailpipe than a healthy engine should make, at idle or when revved, but not enough to be a hazard to other drivers. The emissions test itself was passed. This is the tester telling you what their eyes saw before the meter did.
9.4% fail on smoke at the next test and 12.0% fail on emissions of some kind. Split it by colour and the picture sharpens: the blue smoke wording fails 13.6% of the time, the black smoke wording 3.6%. Blue means the engine is burning oil, and oil consumption only goes one way.
A diesel that smokes black under acceleration and has recently had its warning light on may have a blocked diesel particulate filter or a failed EGR valve. Those get expensive if left, and a DPF that has been removed is an automatic MOT failure.
An “exhaust emits excessive smoke or vapour but not likely to obscure the vision of other road users” advisory means the tester saw more smoke from the exhaust than normal, but not enough to be dangerous, and the emissions test was passed. Blue smoke means burning oil, black means unburnt fuel, white is usually coolant or condensation. According to DVSA MOT data analysed by PriceMyRepair, 9.4% of 153,116 such advisories followed to the next test failed on exhaust smoke.
What each colour is telling you
Blue. Oil is getting into the combustion chamber and burning. On an older petrol engine the usual routes are worn valve stem seals, which give a puff of blue on start-up that clears, and worn piston rings, which give blue under acceleration. On a turbocharged engine a worn turbo seal does the same. “blue smoke” is the most common tester note, and “slight smoke first rev” describes the valve seal case exactly.
Black. Too much fuel, or not enough air. On a diesel it is a blocked air filter, a faulty injector, a lazy EGR valve or a DPF that is no longer trapping soot. On a petrol it is a rich mixture from a failed sensor. Black smoke is usually the cheapest of the three to fix and the one most likely to show up on the emissions meter first.
White. Thin white vapour on a cold morning is water and clears in a mile. Thick white smoke that smells sweet and does not clear is coolant getting into the cylinders, which points to a head gasket. “white smoke” and “when cold” both appear in the notes, and the tester is describing the second kind.
Will it fail next year?
Three wordings are grouped here: the general one and two that name the colour. Together they have been recorded 393,423 times, and 153,116 can be followed to the same vehicle’s next MOT.
| What happened at the next test | Share |
|---|---|
| Failed on the same component | 9.4% |
| Failed on anything under defect 8.2.2.2 | 12.0% |
| Same advisory recorded again | 14.2% |
According to DVSA MOT data analysed by PriceMyRepair: 153,116 advisories followed to the same vehicle’s next test. Rows can overlap.
Read the wordings separately and the colours tell different stories. The blue smoke wording has been used 125,102 times and 13.6% fail on smoke at the next test. The black smoke wording, 48,248 times, fails 3.6%. Black smoke gets fixed, because it is usually a filter or a sensor. Blue smoke is worn metal, and worn metal is not fixed for the price of a service.
Only 14.2% are advised again, low for any advisory. An engine that smokes visibly is either repaired, sold, or fails.
| Advisory wording | Fails on same part | Fails under same code | Advised again | Followed |
|---|---|---|---|---|
| Exhaust emits excessive smoke or vapour | 9.4% | 12.0% | 14.2% | 153,116 |
| Exhaust has a minor leak of exhaust gases | 9.0% | 9.0% | 26.5% | 3,311,485 |
| Exhaust mounting defective | 5.5% | 5.5% | 23.3% | 1,459,563 |
| Engine mounting damaged | 2.2% | 2.2% | 13.6% | 101,134 |
| Oil leak, | 1.2% | 1.3% | 42.8% | 8,941,140 |
| Exhaust noisy | 0.9% | 0.9% | 39.7% | 382,681 |
| Undertray not fully secure | 0.6% | 0.7% | 8.6% | 83,373 |
Nothing to report on cars under seven years old. From 7–9 years the rate rises from 0.4 per 1,000 tests to about three per 1,000 in the late teens. The failure rate is unusual in staying flat, between eight and ten in a hundred at every age. A smoking engine is a smoking engine whatever its birthday.
| Age of car | Per 1,000 tests | Fails on same part | Followed |
|---|---|---|---|
| 7–9 years | 0.4 | 8.6% | 13,474 |
| 10–12 years | 1.2 | 9.6% | 35,028 |
| 13–15 years | 2.4 | 9.8% | 47,458 |
| 16–19 years | 3.2 | 9.5% | 36,984 |
| 20 years and over | 3.1 | 9.4% | 13,773 |
Check the oil level and note it, then check again in five hundred miles. An engine that has used more than a litre is burning it, and the smoke is blue. Look at the coolant level the same way, and at the oil filler cap for a creamy deposit. And on a diesel, look at the air filter, which takes a minute and is often the whole answer to black smoke.
Take the results to the garage. A tester’s advisory plus a measured oil or coolant loss is a diagnosis half done.
A DPF warning light, a smell of oil after a motorway run, or an engine that needs topping up between services are the signs that this advisory is about to become a bill. Do not keep driving a diesel with the DPF light on.
Exact wording on your certificate
The general wording is an advisory when the smoke is not a hazard. The failures are for dense smoke and for a diesel that fails the smoke opacity test.
| Line on the certificate | Result | Code |
|---|---|---|
| Exhaust emits excessive smoke or vapour but not likely to obscure the vision of other road users | Advisory — pass | 8.2.2.2 (c) |
| Exhaust emits blue smoke during acceleration | Advisory — pass | 8.2.1.2 (g) |
| Exhaust emits black smoke during acceleration | Advisory — pass | 8.2.1.2 (g) |
| Exhaust emits dense blue or clearly visible black smoke | Major — fail | 8.2.1.2 (g) |
| Exhaust emits smoke or vapour likely to obscure the vision of other road users | Major — fail | 8.2.2.2 (c) |
Almost every entry has no position, because smoke has one source. The handful marked rear are the tailpipe.
| Position written by the tester | Share of all instances |
|---|---|
| No position recorded | 97% |
| Rear | 3% |
| Nearside rear | 0% |
| Offside rear | 0% |
| Front | 0% |
| Central rear | 0% |
From 393,423 recorded instances. Testers can add their own note after the standard wording. The most common on this advisory: “blue smoke” (284), “slight smoke first rev” (109), “black smoke” (100), “blue” (41), “under acceleration” (33), “on acceleration” (32), “slightly” (31), “when cold” (30).
What it can cost, by colour
There is no single price, because there is no single fault. Black smoke is usually a filter, sensor, injector or EGR job. Blue smoke on a turbo engine may be the turbo; on a non-turbo engine, if it is rings rather than seals, it is an engine rebuild question and a car-value question. Coolant in the exhaust is a head gasket. Each has its own page: turbo replacement cost, EGR valve cost, head gasket repair cost.
Can I still drive with this advisory?
Keep an eye on the oil and coolant levels and do not let either drop low. A diesel with a DPF light should not be driven until it is dealt with, and any engine that suddenly produces thick white smoke should be stopped and checked for coolant loss.
Enter your reg for your car’s full MOT record, including any earlier oil leak or emissions advisories that may be part of the same story.
Which cars get this advisory most?
The all-vehicle rate is 0.6 per 1,000 tests. On the models below it is three to six times that.
| Model | Per 1,000 tests | Vs all cars | Tests |
|---|---|---|---|
| Volkswagen Transporter | 4 | 5.7× | 3,128,645 |
| Fiat Doblo | 3 | 4.8× | 537,931 |
| Mercedes-Benz Vito | 3 | 4.4× | 968,046 |
| Mercedes-Benz Sprinter | 3 | 4.0× | 2,101,182 |
| Vauxhall Combo | 2 | 3.7× | 1,167,848 |
| Land Rover Defender | 2 | 3.3× | 1,232,451 |
| Volkswagen Crafter | 2 | 3.2× | 585,956 |
| Land Rover Freelander | 2 | 3.2× | 1,480,990 |
| Volkswagen Passat | 2 | 3.1× | 2,274,085 |
| Audi A4 | 2 | 3.1× | 2,768,934 |
Model families are grouped by the first word of the DVSA model name, so “Transit” covers every Transit variant. Only models with at least 60,000 tests are ranked.
Seven of the ten are diesel vans and 4x4s: the Volkswagen Transporter at 5.7 times the average, the Fiat Doblo, Mercedes-Benz Vito, Mercedes-Benz Sprinter, Vauxhall Combo, Land Rover Defender and Volkswagen Crafter. High-mileage diesels with turbochargers, EGR valves and particulate filters have more ways to smoke than a small petrol car, and all of them are used hard.
The Volkswagen Passat and Audi A4 share diesel engines across the same group.
What to do next
- Get the colour from the tester. If the certificate does not say, ring the test station. Blue, black or white changes everything that follows.
- Measure the oil use over five hundred miles. A litre or more is burning, and the smoke is oil.
- On a diesel, check the air filter and the DPF light first. The cheap causes of black smoke, and the common ones.
- Get a diagnosis before a quote. Smoke has too many causes for anyone to price it from the certificate.
Exhaust smoke advisory FAQ
What does exhaust emits excessive smoke or vapour mean on an MOT?
The tester saw more smoke from the exhaust than a healthy engine should produce, but not enough to be a hazard to other drivers, and the emissions test was passed. It is an advisory and the car passes.
What does blue smoke from the exhaust mean?
The engine is burning oil. Common causes are worn valve stem seals, giving a puff on start-up, worn piston rings, giving smoke under acceleration, or a worn turbo seal on a turbocharged engine.
What does black smoke from the exhaust mean?
Unburnt fuel. On a diesel it is usually a blocked air filter, a faulty injector, a sticking EGR valve or a DPF problem. On a petrol it is a rich mixture, often from a failed sensor. It is generally the cheapest colour to fix.
Will it fail the next MOT?
According to DVSA MOT data analysed by PriceMyRepair, 9.4% of 153,116 such advisories failed on exhaust smoke at the next test. The blue smoke wording fails more often, at 13.6%, and the black smoke wording less, at 3.6%.
Is it safe to drive with a smoking exhaust?
Yes, as long as oil and coolant levels are kept up. Stop and have the car checked if thick white smoke appears suddenly, and do not drive a diesel with the DPF warning light on.
Which vehicles get this advisory most?
Diesel vans and 4x4s. The Volkswagen Transporter, Fiat Doblo and Mercedes-Benz Vito get it four to six times as often as the average vehicle.
How these figures were calculated. According to DVSA MOT data analysed by PriceMyRepair, from the DVSA anonymised MOT test results (Open Government Licence v3.0): 350,943,111 tests. This advisory was recorded 393,423 times; 153,116 of those were followed to the same vehicle’s next test between 180 and 730 days later. “Fails on the same part” means a failure at that next test under the same defect code and naming the same component, on either side of the vehicle. Testers do not always name a component the same way twice, so the figure is a floor. Method and how to cite.