My 2004 Tahoe threw a P0442 code the other day. 5.3 ltr flex engine. I understand it is a small leak somewhere in the evap system. I visually inspected all the lines to see if I could find something broken, but they all seem to be in order. I don't think it is the gas cap. The cap and seals look good and it feels right when I turn it the way it always has and should. How do you check the vapor canister vent valve and the vapor canister purge valve for proper operation? Should they be open or closed in home position? I inspected the vent valve to see if it was plugged up with dust or dirt, but it was not. Any ideas from experience what it could be? All suggestions are welcomed and appreciated.