Emissions and evaporative
P0456Very small evaporative emission system leak detected
EVAP System Leak Detected (very small leak)
Get it diagnosed soon
Generic OBD-II code
Evaporative Emission (EVAP) System Component
What this part does
Captures and purges fuel vapor from the fuel tank/charcoal canister into the engine rather than releasing it to the atmosphere; includes the purge and vent solenoids and related sensors.
What the computer is measuring
The evaporative system is sealed by design, and the computer tests that seal by pulling the system down and watching whether it holds. There are three leak thresholds. P0455 catches a large one, P0442 a small one, and P0456 the tightest test of the three, aimed at a hole around a fifth of a millimetre across.
At that size there is nothing to hear, nothing to smell and nothing wrong with how the car drives. The only evidence is the light. This is a pure emissions code, and the reason manufacturers test to that threshold at all is that a great many vehicles each leaking a very small amount adds up.
Two practical things follow from how tight the test is. First, the fuel cap seal is a disproportionately common cause, because rubber that has gone slightly hard will pass a casual inspection and still leak this much. Replacing the cap is a reasonable first move rather than a guess.
Second, confirming a repair takes patience. The evaporative monitor only runs under specific conditions, typically with the tank between about a quarter and three quarters full, after the engine has been off long enough to cool, and within a range of ambient temperatures. That can mean several days of ordinary driving before the test runs again and the code either returns or stays away. A repair that looks successful after one trip has not actually been tested yet.
What you will notice
- Steady check engine light and no drivability change whatsoever.
- Frequently appears, clears on its own, and returns weeks later.
- No fuel smell in most cases, because the leak is genuinely tiny.
Likely causes, most common first
Ordered by how often each one actually turns out to be the problem, not alphabetically and not by part price.
A hardened or cracked fuel cap seal
The most common cause. At this leak size the cap does not have to be loose, the rubber seal simply has to have gone hard with age.
A small crack or pinhole in a vapour hose
Usually at a bend, at a clamp, or where a hose has rubbed against something for years.
A purge valve that does not seal completely
A valve leaking slightly when commanded closed lets vapour past continuously, which reads as a very small persistent leak.
A weeping seal at the fuel tank sending unit
Common after a fuel pump replacement, where the large O ring was reused or pinched on refitting.
The start of rust perforation at the filler neck
Catches this code before it grows into P0442 or P0455.
What to check, in order
Replace the fuel cap with the correct part for the vehicle, then clear the code and drive normally for several days.
At this leak size a visual inspection of the seal is not conclusive, and a cap is inexpensive. If the code stays away through several drive cycles, that was it.
Check the fuel level when the code sets, if the vehicle records it.
The evaporative monitor usually runs only within a band of tank levels, roughly a quarter to three quarters full. Understanding when it runs explains why the code seems to come and go.
Trace the vapour hoses looking for chafe marks and hardened sections, paying attention to where they pass near hot or moving parts.
A visible crack explains it. Nothing visible is expected at this size and does not clear the hoses.
Have the system smoke tested at low pressure.
This is the only practical way to find a leak this small. A system that holds smoke points at the purge valve, which leaks only under command.
Where this one turns up most
- Vehicles over about eight years old on the original fuel cap.
- Anything that has recently had a fuel pump or sending unit replaced.
Codes that often show up alongside it
What a DriveDecode adapter can read
A DriveDecode adapter reads engine and emissions systems: the powertrain and emissions codes the OBD-II standard requires on every US vehicle it covers. It does not read ABS, airbag (SRS), TPMS or transmission-specific manufacturer codes. Those need make-specific decoding that generic OBD-II hardware cannot do.
Reading P0456 on your own car
DriveDecode pulls P0456 straight off the vehicle along with the freeze frame the computer stored when it set: the coolant temperature, engine load and speed at that exact moment. That context is what separates a fault that happens on a cold start from one that happens under load, and it is the first thing the list above asks you to narrow down.
Last reviewed 2026-09-05. Definition sourced from the published standard.