2004 Chevy Cavalier P0455 Evap System Leak Detected

you need a scan tool and a smoke machine to really figure it out. but most likely its the vent valve. i would replace that and the gas cap.

I recently had a Chevrolet Sunfire which is the same as the Cavalier with a gross leak detected P0455. I ran a smoke test and found that the leak was coming from the top of the fuel tank where the evap hose connects to the tank. I had to drop the tank and re-install the hose. Cleared the code and never did reset again.