Emissions and evaporative
P0455Large evaporative emission system leak detected
EVAP System Leak Detected - Large 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 emission system captures fuel vapour from the tank in a charcoal canister and later feeds it into the engine to be burned. The whole path has to be sealed for that to work, so the computer periodically tests it by applying a vacuum or a pressure and watching whether it holds.
P0455 is what it sets when the system fails to hold at all, or leaks far faster than the small leak threshold. Where P0442 is looking for a hole around half a millimetre across, this code is triggered by something in the order of a millimetre and a half or larger. That is a big enough opening that you can very often find it by looking, which makes this the most approachable of the evaporative codes.
It also means the fuel cap deserves the first look even more than it does on P0442. A cap left on the roof at the petrol station, one that was never turned until it clicked, or one whose seal has cracked right through will all produce this. Check it before you do anything else, refit it properly, clear the code, and drive the vehicle for a few days to let the test run again.
If the cap is genuinely fine, the next most productive thing is to get underneath with a torch. A leak this size is usually a hose off a fitting or a cracked canister, and both are visible.
What you will notice
- Steady check engine light, most often noticed shortly after filling up.
- A fuel smell around the vehicle, particularly at the rear.
- The car drives normally in every other respect.
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 missing, loose or badly seated fuel filler cap
The most common cause of a large leak by a long way. Check this first, always, before anything else.
A disconnected or split vapour hose
A large leak means a large opening. A hose knocked off during other work, or split right through, produces exactly this.
A vent valve stuck open
The valve that seals the system against atmosphere. Stuck open, the system can never hold pressure and the leak reads as a large one.
A cracked charcoal canister
Mounted under the vehicle on most cars, so it collects road debris and salt and can be broken by a kerb strike.
A rusted or damaged filler neck
A hole large enough to set this code is usually visible once you look at the neck where it enters the tank.
What to check, in order
Check the fuel cap. Remove it, look at the seal, refit it until it clicks, then clear the code and drive.
A code that does not come back means the cap was the problem. If the cap is missing entirely, this is the whole diagnosis.
Look underneath at the charcoal canister and the hoses running to it.
A large leak is often physically visible, whether that is a hose hanging free, a cracked canister body or a broken fitting. Finding one ends the search.
Inspect the filler neck and the tank seam for rust perforation.
Visible perforation explains the code and needs no further testing.
If nothing is visible, have the system smoke tested.
Smoke locates the opening directly. A system that holds smoke points instead at the vent valve, which can leak only while commanded closed.
Where this one turns up most
- Any vehicle immediately after refuelling, where the cap was left loose or on the roof.
- Older vehicles with the canister mounted low and exposed under the rear.
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 P0455 on your own car
DriveDecode pulls P0455 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.