Can a loose fuel cap cause the engine management light?

Yes, and this is the code it sets. The filler cap forms part of a sealed fuel vapour system that the car pressure-tests itself, so a cap left off or not clicked fully home means the system cannot hold any test pressure and a gross leak is recorded. Refit the cap correctly and the lamp will usually go out on its own after several journeys, because the test has to run and pass a number of times before the control unit accepts the fault has gone.

Will P0455 fail an MOT?

Yes, and potentially on two counts. The illuminated engine management lamp is a major defect on petrol cars first used on or after 1 July 2003, which fails the car by itself. Separately, the tester inspects the fuel filler cap, and a missing cap or one that does not seal is recorded as a defect in its own right. Since a faulty or absent cap is the most common cause of this code, both can arise from the same thing.

How long does it take for the light to go off after tightening the fuel cap?

Usually a few days of normal driving, and sometimes up to a couple of weeks. The evaporative leak test does not run on every journey — it needs particular conditions of fuel level, temperature and engine state — and most control units want to see the test pass several times before extinguishing the lamp. If it has not cleared after two weeks of regular use, the cap was probably not the cause and the system needs looking at properly.

Is a large EVAP leak dangerous?

Ordinarily no, because what escapes is petrol vapour in small quantities from a system that operates at very low pressure. It becomes a genuine concern if the cause is a split in the filler hose or a damaged tank rather than the cap, because those can release liquid fuel. Any strong petrol smell, a smell that reaches the cabin, or wetness under the car should be treated as a fire risk and looked at before the car is driven again.

Powertrain code · standard

P0455Large leak in the fuel vapour system

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

Last updated 27 July 2026

Quick answer

The fuel system could not build or hold any test pressure at all, which means a big opening somewhere. A filler cap left off, not clicked home or lost entirely is by far the most common explanation, and it is the reason every piece of advice about the engine light starts with checking the cap.

How the car works this out

The control unit runs the same sealed-system test as for a small leak, but here the result is unambiguous. Rather than measuring a decay rate against a threshold, it finds that pressure or vacuum barely develops in the first place. Something is open to atmosphere on a scale that no small hole would explain: a cap off, a hose detached, a vent valve stuck open, or a pipe split wide. Because the fault is so clear-cut, it usually appears sooner after the event than the small-leak codes do.

What you might notice

The lamp appearing within a day or two of filling up
A noticeable petrol smell around the car, particularly at the rear
Nothing at all in the way of driving symptoms
On some cars, a fuel gauge that behaves oddly if a tank breather is involved

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.

1The fuel filler cap left off, not clicked home, or replaced with the wrong one after being lostCommon
How to check: Open the flap and look. This takes seconds, costs nothing, and is genuinely the answer a great many times.
2A hose detached, split wide open, or perished through around the charcoal canister or filler neckCommon
How to check: Get the car up and follow the pipework. A leak of this size is usually visible rather than needing smoke.
3A canister vent valve stuck open, so the system can never be sealed for the testPossible
4A corroded or damaged filler neck, or a split filler hose, which is common on older cars in a salted-road climatePossible
5A cracked or broken charcoal canister, or a broken outlet spigot snapped off during other work underneathPossible
6A leaking seal at the fuel pump or sender flange on top of the tankLess common

What to do about it

1Check the cap first, and check it properly — off, seal inspected, back on until it clicks positively. If it was the cause, the lamp will clear itself after several drive cycles without any intervention.
2If the cap is sound, get the car up and look. Unlike the small-leak codes, a gross leak is usually large enough to find by eye and touch.
3Take a persistent petrol smell seriously. Vapour escaping is one thing; a smell strong enough to notice in the cabin or a wet patch under the car is a fire risk.
4Do not simply clear the code and hope. If the leak is real it will set again, and clearing it wipes the readiness monitors at the worst possible moment before a test.
5If the cap has been lost, buy the correct part rather than a universal one. The seal profile and the pressure-relief behaviour are specific to the car.

Can you keep driving?

Yes

Vapour escaping from the evaporative system does not affect how the car drives or stops. The judgement call is whether what is escaping is really only vapour. A strong petrol smell, a smell inside the cabin, or any liquid under the car moves this from an emissions nuisance to a fire risk, and at that point it should be looked at before the car is used again.

Will it fail an MOT?

This is the code behind the advice on every engine light page to check the fuel cap before doing anything else, and it deserves saying plainly: a cap that has simply not been clicked home can cost somebody an MOT, because the lamp it lights is a major defect on petrol cars first used on or after 1 July 2003. The tester also examines the fuel filler cap directly, and a missing cap or one that will not seal is recorded as a defect on sight — so the cheapest and most avoidable fault in this entire module is also one of the few that can fail a car by two separate routes at once. If the lamp came on shortly after a fill-up, refit the cap and drive the car for several days before the test rather than booking it straight in.

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

Can a loose fuel cap cause the engine management light?: Yes, and this is the code it sets. The filler cap forms part of a sealed fuel vapour system that the car pressure-tests itself, so a cap left off or not clicked fully home means the system cannot hold any test pressure and a gross leak is recorded. Refit the cap correctly and the lamp will usually go out on its own after several journeys, because the test has to run and pass a number of times before the control unit accepts the fault has gone.
Will P0455 fail an MOT?: Yes, and potentially on two counts. The illuminated engine management lamp is a major defect on petrol cars first used on or after 1 July 2003, which fails the car by itself. Separately, the tester inspects the fuel filler cap, and a missing cap or one that does not seal is recorded as a defect in its own right. Since a faulty or absent cap is the most common cause of this code, both can arise from the same thing.
How long does it take for the light to go off after tightening the fuel cap?: Usually a few days of normal driving, and sometimes up to a couple of weeks. The evaporative leak test does not run on every journey — it needs particular conditions of fuel level, temperature and engine state — and most control units want to see the test pass several times before extinguishing the lamp. If it has not cleared after two weeks of regular use, the cap was probably not the cause and the system needs looking at properly.
Is a large EVAP leak dangerous?: Ordinarily no, because what escapes is petrol vapour in small quantities from a system that operates at very low pressure. It becomes a genuine concern if the cause is a split in the filler hose or a damaged tank rather than the cap, because those can release liquid fuel. Any strong petrol smell, a smell that reaches the cabin, or wetness under the car should be treated as a fire risk and looked at before the car is driven again.

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