Doe's your rig have an evaporator canister and tubing in your smog control system? we had a rig that coded replace cap. We did the cap cleared the code. A week later the cap code came back. After research found out canister tubing leakage also activates cap code. Found crack in numerous spots on the tubing. replaced tubing no problems since. Possibly your getting a new cap, code clears and you don't look for this code again until smog check time. Maybe be something to look at Good luck