P0455: EVAP System Leak Detected (Large Leak)
P0455 means the car's evaporative emission (EVAP) system, which traps fuel vapor from the tank, has a large leak or cannot build vacuum. It rarely affects driveability and is often fixed by tightening or replacing the fuel cap, but it will keep the check engine light on and can fail an emissions test.
You can keep driving
The code does not cause limp mode and the engine normally runs as usual. A large leak can let fuel vapor escape, so fix it soon, and have it checked right away if you smell fuel strongly.
What it means
The EVAP system stores fuel vapor from the tank in a charcoal canister instead of letting it escape into the air. A purge valve lets the engine draw that vapor off to burn it, and a vent valve opens the canister to fresh air. To test the system, the engine computer closes it off and tries to pull a small vacuum, or watches for a pressure change. P0455 is set when that test shows a large leak or no vacuum builds. The code also covers blocked purging, such as a purge valve stuck closed. Thresholds differ by manufacturer. Ford, for example, expects about −2 kPa within 30 seconds, and after refueling it sets P0457 instead. GM counts purged air volume against a tank vacuum target, and Saab looks for a pressure drop below a set level. The light usually turns on after one or two trips, depending on the maker.
Likely causes
- Fuel cap missing, not clicked in, or with a faulty seal (or a capless filler flap that doesn't seal)Very likely
The cap seals the tank, so if it is loose or its seal is worn, the system cannot hold vacuum and the computer sees a large leak.
- Hose off or cracked (canister, filler neck, or purge line)Very likely
A disconnected or split vapor hose opens the sealed system to outside air, which is a large leak.
- Vent valve stuck open or purge valve stuck closedPossible
A vent valve stuck open leaks air in, and a purge valve stuck closed stops vacuum from reaching the tank, so the test fails either way.
- Cracked or corroded canister or filler neckPossible
Damage or rust creates an opening in the system's hard parts that hoses and caps cannot fix.
What you may notice
- Check engine light on, possibly with a check fuel cap message
- Possible fuel smell
- EVAP readiness monitor not complete, or a failed emissions inspection
What to check, cheapest first
- 1Check that the fuel cap is present, clicked in fully, and that its rubber seal is not cracked or dirty. On a capless filler, check that the flap closes properly. · you can do it
- 2Replace the fuel cap if it is damaged or worn, then clear the code and drive normally to see if it returns. The system needs a drive cycle (a set pattern of driving) to retest. · you can do it
- 3Look at the visible hoses near the filler neck, canister, and purge line for disconnected, split, or loose ends. · you can do it
- 4Inspect the filler neck and canister for cracks or rust. · mechanic
- 5Test the purge valve and vent valve to see whether they open and close as commanded and hold vacuum. · mechanic
- 6Run a smoke test, which pushes harmless smoke into the system to show where the leak is, and repair what it finds. · mechanic
Related codes
Frequently asked questions
Can I drive with P0455?+
Yes, in most cases. The engine is not put into limp mode, but vapor may be escaping, so repair it soon.
Can a loose gas cap cause P0455?+
Yes. A missing, loose, or worn fuel cap is one of the most common causes. Replace or tighten it, clear the code, and drive to see whether it returns.
Will P0455 pass an emissions test?+
Usually not. The code is emissions-related and the check engine light is on, so the leak should be fixed and the EVAP monitor completed first.
How hard is it to fix?+
Often easy and doable at home, such as a new fuel cap or hose. Repairs take from a few minutes to about 3 hours depending on the cause.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10