Will P0442 fail an MOT?

Yes. The engine management lamp is a major defect on petrol cars first used on or after 1 July 2003, so the car fails on the lamp alone. There is no evaporative emissions measurement in the MOT, so nothing in the test can show the fault is minor. The tester does check the fuel filler cap separately, and since a failed cap seal is the most common cause of this code, a car can end up with two recorded defects from the same underlying problem.

Can a loose petrol cap cause a P0442 code?

Yes, and it is the first thing to check. The cap is part of the sealed fuel system, so a cap that has not been clicked home, has a hardened or split seal, or is simply the wrong cap for the car will let the system fail its own leak test. Refit it properly, drive for a few days, and see whether the lamp goes out. The test does not run on every journey, so give it several drive cycles before concluding the cap was not the cause.

How do you find a small EVAP leak?

With a smoke machine. The system is filled with a harmless dense vapour at very low pressure and the escape point is then visible. A leak of the size this code describes is roughly a millimetre across and can be anywhere between the filler cap, the tank flange seal, the canister and metres of hose running under the car, so searching visually is impractical. Any garage doing evaporative diagnostics properly will have the equipment, and the test takes minutes rather than hours.

Is it safe to drive with an EVAP leak?

For a small leak, yes. What is escaping is petrol vapour rather than liquid, in small quantities, and nothing about how the car runs or stops is affected. The exceptions are a strong persistent smell of fuel or any sign of wetness or dripping underneath the car — those indicate liquid fuel escaping, which is a fire risk and needs looking at straight away rather than at the next service.

Powertrain code · standard

P0442Small leak in the fuel vapour system

Standard definition: Evaporative Emission System Leak Detected (Small Leak)

Last updated 27 July 2026

Quick answer

The car has pressure-tested its own fuel system and found a leak roughly the size of a pinhole about a millimetre across. Petrol vapour is escaping to atmosphere somewhere between the tank and the charcoal canister. The fuel filler cap is the first thing to check and quite often the whole answer.

How the car works this out

The control unit periodically seals the fuel system and tests it for leaks. Depending on the design it either closes the canister vent valve and uses manifold vacuum through the purge valve to draw the system down, then watches how quickly the vacuum decays, or it pressurises the system with a small dedicated pump and measures the same thing. The decay rate is compared against the profile of a hole of a known size — for this code, around a millimetre across. The test only runs when conditions are right, typically with a part-full tank, stable temperatures and the engine settled, which is why the lamp can come and go over several weeks.

What you might notice

Nothing you would notice while driving
A faint smell of petrol near the rear of the car, particularly after a warm day
The lamp appearing a day or two after refuelling and sometimes clearing itself

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1A fuel filler cap that is not sealing — the wrong cap, a perished rubber seal, or one that has not been clicked fully homeCommon
How to check: Look at the seal for hardening, splits or a flattened profile, and check the cap clicks positively. Refit it properly and drive several journeys before assuming it is not the cause.
2Perished or split rubber hoses and elbows around the charcoal canister, which lives under the carCommon
How to check: A smoke test through the sealed system shows the escape point immediately. Trying to find a millimetre-wide leak by eye is largely a waste of an afternoon.
3A purge or vent valve not sealing fully when closed, so the system can never hold its test pressurePossible
4A failed seal on the fuel pump or sender unit flange in the top of the tankPossible
5A damaged or corroded filler neck, or a split in the filler hosePossible
6A cracked charcoal canister, often from stone damage or corrosion of its mountingsLess common

What to do about it

1Start with the cap. Take it off, look at the seal, refit it until it clicks properly and drive for several days. It costs nothing, and it resolves a meaningful proportion of these.
2If the cap is not the answer, ask for a smoke test rather than a parts-swapping exercise. A leak of this size is found in minutes with the right equipment and can be hunted for hours without it.
3Look underneath at the canister and its hoses while the car is up. Rubber down there hardens and cracks in the British climate, and a visual check often finds it.
4Be patient after a repair. The leak test does not run on every journey, so the lamp may take several drive cycles to go out even when the fault is genuinely fixed.
5Never introduce a naked flame or a pressure source of your own to look for it. This is petrol vapour.

Can you keep driving?

Yes

A small evaporative leak has no effect on how the car runs and does not make it unsafe. The material escaping is vapour, not liquid, and the quantity is small. The one thing to take seriously is a strong smell of petrol or any sign of wetness underneath, because that means liquid fuel and a different order of problem entirely.

Will it fail an MOT?

This is one of the very few emissions codes where the tester can find something with their own eyes rather than relying on the lamp. The fuel filler cap is inspected as part of the test, and a missing cap or one that does not seal properly is recorded as a defect independently of anything stored in the control unit — so a car with this code caused by a failed cap seal can be failed twice over from the same root cause. A fuel leak from the system is more serious again and is treated accordingly. Beyond that the usual rule applies: the illuminated lamp is a major defect on petrol cars first used on or after 1 July 2003, and there is no evaporative measurement in the MOT to contradict or confirm it.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Will P0442 fail an MOT?: Yes. The engine management lamp is a major defect on petrol cars first used on or after 1 July 2003, so the car fails on the lamp alone. There is no evaporative emissions measurement in the MOT, so nothing in the test can show the fault is minor. The tester does check the fuel filler cap separately, and since a failed cap seal is the most common cause of this code, a car can end up with two recorded defects from the same underlying problem.
Can a loose petrol cap cause a P0442 code?: Yes, and it is the first thing to check. The cap is part of the sealed fuel system, so a cap that has not been clicked home, has a hardened or split seal, or is simply the wrong cap for the car will let the system fail its own leak test. Refit it properly, drive for a few days, and see whether the lamp goes out. The test does not run on every journey, so give it several drive cycles before concluding the cap was not the cause.
How do you find a small EVAP leak?: With a smoke machine. The system is filled with a harmless dense vapour at very low pressure and the escape point is then visible. A leak of the size this code describes is roughly a millimetre across and can be anywhere between the filler cap, the tank flange seal, the canister and metres of hose running under the car, so searching visually is impractical. Any garage doing evaporative diagnostics properly will have the equipment, and the test takes minutes rather than hours.
Is it safe to drive with an EVAP leak?: For a small leak, yes. What is escaping is petrol vapour rather than liquid, in small quantities, and nothing about how the car runs or stops is affected. The exceptions are a strong persistent smell of fuel or any sign of wetness or dripping underneath the car — those indicate liquid fuel escaping, which is a fire risk and needs looking at straight away rather than at the next service.

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