P0455 came back after a new gas cap

What the computer saw

First, check whether the light came back or never went off. After a fix, the computer must rerun its leak test, which on many cars takes several days of normal driving. A code that truly returns means the leak is somewhere else.

Likely causes, most common first

1. The test has not rerun yet. The light can stay on for days after the fix while the computer waits for the right conditions.
2. A disconnected or cracked EVAP hose. The most common cause once the cap is ruled out.
3. A purge or vent valve stuck open. It looks like a big leak to the test.
4. A cap not made for the car, or not clicked tight. A loose fit leaks like no cap.
5. A cracked canister or filler neck. Found with a smoke test.

What you can note yourself

• Make sure the cap clicks when tightened, and that it is the type made for your car.
• Note whether the light went off at all after the new cap.

What to tell the shop

• When the cap was replaced, and whether the light ever went off.
• Ask for a smoke test of the system.

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

Related pages

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