P0455: a large leak in your fuel vapor system

What the computer saw

Your fuel tank's vapors are held in a sealed system and burned in the engine instead of escaping into the air. The computer seals that system now and then and tests whether it holds. P0455 means it found a large leak, about the size of a missing gas cap or a disconnected hose.

Likely causes, most common first

1. A loose, missing or worn gas cap. The most common cause by far, and the first thing to check.
2. A disconnected or cracked EVAP hose. Hoses between the tank, the charcoal canister and the engine can crack or pop off.
3. A purge or vent valve stuck open. These valves open and close the system. One stuck open looks like a big leak.
4. A cracked charcoal canister or filler neck. Less common; a shop finds it with a smoke test.

What you can note yourself

• Take the gas cap off, look at its rubber seal for cracks or tears, and put it back on until it clicks.
• If the cap is damaged or was missing, replace it with one made for your car.
• Give it time. On many cars the computer reruns the leak test only under certain conditions, such as a cold start with the tank between about a quarter and three quarters full, so the light can stay on for several days after the fix.

What to tell the shop

• All the codes, and whether P0442 or P0456 came with it.
• Whether you already checked or replaced the gas cap, and when.
• Ask whether they will smoke-test the system before replacing valves.

This page gives no verdict on whether a car is safe to drive; a mechanic can.

Related pages

• P0442 and P0455 together: a small and a large fuel vapor leak
• P0442: a small leak in your fuel vapor system
• P0455 came back after a new gas cap