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.

Those valves are tested by activation and with a vacuum gauge attached. Need to be sure they do not leak.

If the gas cap isn't fairly new, odds are good it'* the gas cap. It can click and seal nicely, but the valve on the inside could still leak. Before I'd go through a bunch of other stuff, I'd replace the gas cap and see what happens.

I remember back to when we had air care/mandatory emission testing.
They had a gas cap pressure tester and told me car was a fail, due to the gas cap.
Luckily I was smart enough to have a spare, and handed the new cap over for testing & sure enough it passed.

Carfixer, what is the normal state of those valves when NOT activated, with other words engine turned off, no keys in the ignition. Should they be open or closed?
CathedralCub, Soft Ride, hmmm, very interesting. I will certainly take your advise and check or replace the gas cap first before I move on to the more complicated stuff.

Thanks y'all

The vent valve is NO and the purge is NC.

The problem has not been resolved yet. I was just too busy in the last month to get to it. But here is what I did so far. I had my son, who vapes, blow a lot of smoke thru the AVAP service connector right next to the vapor canister PURGE valve on top of the engine (the one with the green cap on and the left threaded Schraeder valve in it (for crying out loud why is this thing left threaded. I almost busted the whole thing trying to get it out until I figured out that it was a lefty.)), but that did not show up any leaks anywhere. I figured he may not be blowing hard enough so I actually went and bought a small smoke making machine which worked like a charm, but there were NO leaks to be found anywhere including the gas cap. The only place where smoke came out was at the end of the line at the vapor canister VENT valve. I understand that the PURGE valve is normally closed (engine not running) and the VENT valve is normally open (engine not running). Can anybody explain to me what the function is of these 2 valves and how they are supposed to be doing whatever they are supposed to be doing while driving. Is there anything else that could cause a code P0442 other than the AVAP lines and these 2 valves?

You'll likely not find a small leak with smoke. It should be charged with nitrogen.

Since I didn't see any smoke leaks I will assume for the time being that all the tubing and lines are ok. The 3 suspect parts (purge valve, went valve, gas cap) all OEM are on order from Rock Auto totaling $55 and should be here by the end of the week. Will then replace them one by one and see which fixes the problem, if it does fix the problem. If not back to the think tank. Will keep you all posted. Thanks for all the great info.

Not a good assumption. May be minor cracks in the canister. Maybe a joint that seals most of the time but leaks with vibration while driving which is when the monitor runs. Small leaks are best tested with nitrogen. It will find the little gremlins that smoke does not find.