P0442, P0455 and P0456: EVAP leaks (small, large, very small)

P0442, P0455 and P0456 all mean the same thing at different sizes: petrol vapour is escaping from the sealed fuel system. P0455 is a large leak, P0442 a small one, P0456 a very small one. The commonest cause of all three is a fuel cap that is not sealing. Indicative UK repair cost is £15 to £300.

A note on wording, because it matters when you are searching. Almost everything written about these three codes says "gas cap", because almost everything written about these three codes is American. On your car it is the fuel cap, or the petrol cap, and on a diesel the same system is not fitted at all, so these codes belong to petrol cars. Checking the cap properly is free, and on a good number of cars it is the whole repair.

Can I drive with P0442, P0455 or P0456?

Usually OK to drive, get it checked soon. None of these three is a mechanical risk. Nothing is overheating, nothing is wearing out faster, and the engine is not being damaged while the code is stored. What you lose is fuel vapour to the atmosphere instead of into the engine, a warning light you cannot see past, and, with a large leak, sometimes a smell of petrol around the car. The reason to deal with it is the MOT and the fact that the light will hide the next code that appears behind it.

What these codes mean for your MOT

This is where honest reporting matters, because an EVAP leak is not the MOT problem that an emissions fault like P0420 is.

  • The leak itself is not measured. The petrol emissions test reads carbon monoxide, hydrocarbons and lambda at the tailpipe. A vapour leak at the tank or in a hose does not appear in any of those numbers.
  • The warning light does count. Section 8.2.1.2 of the DVSA MOT inspection manual makes a light that is on and indicating a malfunction a major defect, on petrol cars and petrol hybrids first used on or after 1 July 2003. If one of these codes has turned your light on, that alone is a fail.
  • The fuel cap is its own MOT item. Section 6.1.3 of the same manual lists a "missing or ineffective filler cap" as a major defect in the fuel system. So the cheap part that usually sets these codes is checked at the MOT whether a code is stored or not, which is a good reason to sort it rather than clear it.

The three codes, and what separates them

They are the same test, reported at three sensitivities. The engine computer seals the fuel system and watches whether it holds pressure or vacuum. How fast it loses it decides which code you get.

P0455, EVAP system large leak

A larger vapour leak in the fuel system. Start with the fuel cap, then the EVAP hoses and valves. Large means the system barely held at all, so look for something obvious: a cap left off or cross-threaded after filling up, a vapour hose knocked off, or a vent valve stuck open. Indicative cost £15 to £300.

P0442, EVAP system small leak

A small leak in the fuel-vapour system, very often just a loose or worn petrol cap. The system held, but not well enough. A cap seal that has gone hard, or a cracked hose, is typical. Indicative cost £15 to £250.

P0456, EVAP system very small leak

A tiny vapour leak, commonly a fuel cap not sealing or a hairline crack in an EVAP line. This is the hardest of the three to find by eye, because the leak is small enough that nothing looks wrong. It is the one most worth a smoke test rather than guesswork. Indicative cost £15 to £250.

What usually causes all three

  1. The fuel cap. Loose, cross-threaded, worn, or a pattern cap that does not seal as well as the original. This is first on the list for all three codes, and it is the only one you can check in the next thirty seconds.
  2. A cracked or split EVAP hose. The vapour lines run from the tank forward and are usually plastic. They go brittle with age and heat, and they get caught by jacks, trolley jacks and anything else that goes under the back of a car.
  3. A purge or vent valve at fault. Stuck open, it will not seal and the system cannot build pressure. A purge valve stuck open has a signature worth knowing: the car is awkward to start right after you have filled up, or runs rough for the first minute, because it is pulling raw vapour in while the engine is already rich.
  4. The charcoal canister or its seals. Less common, and more likely on an older car or one that has been stood for a long time, where corrosion and rodent damage both turn up around the rear of the vehicle.

What it costs to fix in the UK

£15 to £300 across the three. A genuine fuel cap is the £15 end. A hose or a purge valve is normally under £150 fitted. The upper end is a canister or a leak that needs real time to find, and P0455 sits slightly higher than the other two because a large leak is more often a component than a cap.

A smoke test is the only item on that list that needs a workshop, so it is worth weighing against what a car diagnostic costs in the UK before you book one.

These are indicative UK ranges for guidance only. Actual causes and prices vary by vehicle, and a stored code points to a system, not always the exact part.

What to check first

  1. Take the fuel cap off and look at it. The rubber seal should be present, soft and unbroken. Wipe the filler neck clean, then refit the cap until it clicks properly home. Drive the car for a few days afterwards. On many cars the code clears on its own once the system holds again.
  2. If the cap is not the original, try one that is. Cheap replacement caps are a common cause of P0456 in particular, and a genuine cap is the cheapest part on the whole list.
  3. Follow the vapour lines by eye. From the tank forward, looking for a split, a disconnected pipe or a line that has been crushed. Pay attention to anything near a jacking point.
  4. Check the behaviour just after refuelling. Hard starting or a rough first minute after a fill-up moves the purge valve to the top of the list instead of the cap.
  5. Then, and only then, pay for a smoke test. A smoke test on the sealed system is the reliable way to find a very small leak, and it is a short job. It is also money wasted if the cap was the problem all along, which is why it comes last.

Related codes

Close by in the same family, P0440 covers a general fault in the EVAP system and P0441 an incorrect purge flow rather than a leak.

Get it narrowed down before you spend

A stored code tells you which system is unhappy, not which part has failed. If you want it narrowed down before you spend anything, send the code and your symptoms and a mechanic will read them personally. Quick Diagnosis is £20, and if we can't help, you don't pay.

Written by the First Diagnostic mechanic. Over a decade in the trade: a VW-only workshop running diagnostics and electrical repair, then BMW-only through to full engine rebuilds. Now an independent all-makes workshop, plus building and wiring drift cars for competition, including trackside work as head mechanic. More about who reads your codes.