P0455 — Evaporative Emission System Leak Detected (Large Leak)
P0455 means fuel vapour is escaping instead of being captured and burned. It does not affect how the car drives and it is not urgent, but it will fail an emissions test — and the most common cause is a fuel cap that is not sealing.
The system has no role in running the engine, so the vehicle drives normally. You may notice a fuel smell, and the fault will not clear itself until the leak is sealed and the monitor re-runs. This code will cause an emissions test failure.
What P0455 means
The evaporative emission system captures fuel vapour from the tank and feeds it into the engine to be burned rather than vented. The module tests the system periodically by sealing it and watching for a pressure or vacuum change. When the loss is too fast to be a small leak, it sets P0455. Because the fuel filler cap is the largest removable seal in that system, it is the first thing to check and the cause in a large share of cases.
| System | Emission control |
|---|
Symptoms you may notice
What causes P0455
Fuel cap loose, cross-threaded or perished
The cap carries a rubber seal that hardens with age and heat. A cap left loose after refuelling, threaded in crooked, or simply old will leak far more than the threshold for a large leak. This is the cheapest possible repair and the most common cause, which is why it is checked first.
Cracked or disconnected vapour hose
The lines running between the tank, the charcoal canister and the engine are plastic and rubber, exposed underneath the vehicle. They harden, crack at bends, and are occasionally knocked loose during other work.
Purge valve stuck open
The purge valve should stay closed while the system is being tested. Held open by debris or a failed solenoid, the system cannot hold pressure and reports a large leak even though nothing is physically broken.
Common misdiagnosis
A stuck purge valve can also lean the idle. If P0455 arrives with lean codes, suspect the valve rather than the cap.
Charcoal canister or vent valve failure
The canister stores vapour until the engine can burn it. A cracked housing, or a vent valve that will not close, leaves the system permanently open. Less common than the cap or the hoses, and the most expensive of the four.
How to diagnose it
Check the fuel cap before anything else
Remove the cap and inspect the rubber seal for cracks, flat spots or hardening. Refit it and tighten until it clicks several times. Clear the code and drive normally for a few days so the monitor can re-run.
Inspect the vapour lines and canister
With the vehicle safely raised, follow the lines from the filler neck to the canister and forward to the engine. Look for cracks at bends, chafing where lines pass brackets, and any connector sitting loose.
Safety
Never work under a vehicle supported only by a jack. Do not use an open flame or a heat source anywhere near the fuel system.
Command the purge valve and listen
Use a scanner with bidirectional control to open and close the purge valve, listening for the click of the solenoid. A valve that does not close, or does not respond at all, cannot hold the system sealed.
Smoke test the system
Introduce low-pressure smoke into the evaporative system through the service port and watch for where it escapes. This finds leaks that inspection misses, including cracks that only open when the plastic is warm.
Related codes
Common questions
Is it safe to drive with P0455?
Yes. The evaporative system has no role in running the engine, so the car drives normally. It will fail an emissions test, and you may smell fuel, so it is worth fixing rather than ignoring.
How long until the light goes out after tightening the cap?
The module only re-runs the leak test under specific conditions — typically a cold start with the tank between roughly a quarter and three quarters full. That can take several days of normal driving.
Why does the code say large leak?
The module measures how fast the sealed system loses pressure. A fast loss is reported as a large leak, which points at an open path such as a missing cap seal, rather than the pinhole that produces a small-leak code.
Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)
This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.