What is the difference between P0442 and P0456?

Only the size of leak the control unit has detected. P0442 corresponds to a hole of roughly a millimetre, P0456 to one of around half a millimetre. The causes overlap almost entirely — cap seals, perished hoses, valve seats and tank flange seals — but P0456 is markedly harder to find because the leak is smaller, and its test conditions are narrower, so it appears intermittently. In practice P0456 is the one that most needs a smoke machine rather than an inspection.

Why does my EVAP code keep coming back and going away?

Because the leak test itself only runs occasionally. Most systems will not attempt the very small leak test unless the car has stood long enough for the fuel to settle, the tank is somewhere between roughly a quarter and three quarters full, and the ambient temperature is within a workable range. On top of that, rubber seals shrink slightly as they get colder, so a marginal leak can genuinely open up on a cold morning and close again in mild weather. An intermittent lamp is normal behaviour for this code, not evidence the fault has fixed itself.

Can I fix a P0456 myself?

You can try the cheap end. Replacing the filler cap, or its seal, is straightforward and accounts for a fair share of these. Inspecting the rubber hoses around the charcoal canister for cracks is also worth doing if you can get the car safely up. Beyond that, finding a half-millimetre leak realistically requires a smoke machine, and hunting for it by replacing parts in sequence tends to cost considerably more than paying for the diagnosis. Never use compressed air or anything that could ignite vapour to search for it.

Powertrain code · standard

P0456Very small leak in the fuel vapour system

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

Last updated 27 July 2026

Quick answer

The smallest leak the car is capable of detecting — around half a millimetre, smaller than a pin. It changes nothing about how the car drives and yet it fails an MOT outright. It is also the hardest of the evaporative faults to find without proper equipment, and the one where parts-swapping most reliably wastes money.

How the car works this out

The same sealed-system test as the other leak codes, run against the tightest threshold the system has. The control unit seals the fuel system, applies vacuum or pressure, and measures the decay against the profile of a hole around half a millimetre across. Because the margin is so fine, the test is fussy about when it will run at all: many vehicles only attempt it after the car has stood long enough for the fuel to settle, with the tank somewhere between roughly a quarter and three quarters full, and within a fairly narrow ambient temperature range. That is why this code can appear once and then not again for weeks, and why it is notorious for coming back in cold weather.

What you might notice

None whatsoever in the way the car drives
Very occasionally a faint petrol smell after the car has stood in the sun
A lamp that comes on, goes off by itself, and returns weeks later

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 hardened or slightly deformed seal on the fuel filler cap, which is the cheapest possible cause and worth ruling out firstCommon
How to check: Run a fingernail round the seal looking for hardening, flat spots or fine cracks. Rubber that has lost its springiness will pass a small-leak threshold and fail this one.
2Fine cracks or permeation in ageing rubber elbows and hoses at the charcoal canisterCommon
How to check: A smoke test is realistically the only way to find these. They are frequently invisible until the vapour shows where it is escaping.
3A vent or purge valve whose seat no longer seals perfectly, letting the system bleed down slowlyPossible
4Pinhole corrosion in a metal vapour line running under the car, which the salted winter roads in this country cause more than most climatesPossible
5A slightly leaking seal at the fuel pump or sender flange on the tankPossible
6A failed leak detection pump on systems that use one, reporting a leak that is not thereLess common

What to do about it

1Replace the filler cap seal or the cap before anything else. It is the least expensive step and it accounts for a real share of these.
2Go straight to a smoke test after that. This is the one code in the evaporative family where trying to save the diagnostic charge by guessing at parts almost always costs more than the diagnosis would have.
3Do not be surprised if the fault is intermittent. The test conditions are narrow, so a car can pass for weeks and then set the code on the first cold morning of the autumn.
4Keep the tank between roughly a quarter and three quarters full for a few days after a repair, since many cars will not attempt the test outside that range and the lamp cannot clear until it has run.
5Ask specifically whether the readiness monitors have completed before booking an MOT after this repair.

Can you keep driving?

Yes

Nothing about this fault affects how the car runs, how it performs, what it consumes or how safely it stops. The quantity of vapour escaping through a half-millimetre hole is very small. The entire practical consequence is the lamp on the dashboard and what that lamp does to the MOT.

Will it fail an MOT?

There is no emissions code with a wider gap between how little it matters mechanically and how completely it fails the MOT. A car with this fault will produce a perfectly clean gas printout, drive faultlessly, and still be issued a failure certificate because the illuminated lamp is a major defect on petrol cars first used on or after 1 July 2003. It is also the code most likely to tempt somebody into clearing it in the car park, which backfires specifically here: the evaporative monitor is among the slowest of all the readiness monitors to complete, needing the right fuel level, the right temperature and a proper soak period, so a car cleared on the morning of the test will very likely arrive with monitors incomplete and fail on an emissions test that could not be finished instead.

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 P0456 fail an MOT even though the car drives fine?: Yes. The MOT contains no evaporative emissions measurement, so nothing in the test can demonstrate how minor this fault is, and the illuminated engine management lamp is a major defect on petrol cars first used on or after 1 July 2003. The car fails on the lamp with an otherwise clean gas printout. Cars first used before that date are not failed on the lamp and would ordinarily pass with this fault stored.
What is the difference between P0442 and P0456?: Only the size of leak the control unit has detected. P0442 corresponds to a hole of roughly a millimetre, P0456 to one of around half a millimetre. The causes overlap almost entirely — cap seals, perished hoses, valve seats and tank flange seals — but P0456 is markedly harder to find because the leak is smaller, and its test conditions are narrower, so it appears intermittently. In practice P0456 is the one that most needs a smoke machine rather than an inspection.
Why does my EVAP code keep coming back and going away?: Because the leak test itself only runs occasionally. Most systems will not attempt the very small leak test unless the car has stood long enough for the fuel to settle, the tank is somewhere between roughly a quarter and three quarters full, and the ambient temperature is within a workable range. On top of that, rubber seals shrink slightly as they get colder, so a marginal leak can genuinely open up on a cold morning and close again in mild weather. An intermittent lamp is normal behaviour for this code, not evidence the fault has fixed itself.
Can I fix a P0456 myself?: You can try the cheap end. Replacing the filler cap, or its seal, is straightforward and accounts for a fair share of these. Inspecting the rubber hoses around the charcoal canister for cracks is also worth doing if you can get the car safely up. Beyond that, finding a half-millimetre leak realistically requires a smoke machine, and hunting for it by replacing parts in sequence tends to cost considerably more than paying for the diagnosis. Never use compressed air or anything that could ignite vapour to search for it.

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