Gas Cap

The removable cap that seals the fuel filler neck and serves as the first line of defense in the EVAP system. A properly sealing gas cap prevents fuel vapors from escaping the tank and maintains the pressure balance the ECU monitors during leak tests. A loose, cracked, or missing cap is the single most common cause of EVAP-related check engine lights — codes P0440, P0442, and P0456 frequently trace back to a $10 cap. Arizona's heat accelerates gasket deterioration on gas caps, and David Jr. recommends replacing the cap as a first step before pursuing more expensive EVAP diagnostics. If tightening or replacing the cap doesn't clear the code after two drive cycles, the leak is elsewhere in the system.