All,
So my 2007 Silverado had a check engine light. P0446/Evap vent circuit. I have done the following:

1.) Tested vent valve solenoid 25 cycles with a 12V battery charger. Functioned correctly.
2.) Tested purge valve solenoid 25 cycles. Functioned correctly.
3.) Removed hose to vent valve. Blew into it. Held thumb over it for 60s. Air whooshed back out.
4.) Gas cap gasket looks good. I put a little silicon oring grease around it just in case.

I replaced the vent valve solenoid thinking it might be failing intermittently. Still getting the P0446. The system does not seem to have any leak, unless it is very minor (or I would have thought the pressure I blew into it would have bled off).

I am guessing it must be the pressure sensor on top of the gas tank. I don't have a scanner to read real time values on that, though. I read that the WIFI ELM 327 OBD (about $20 on Amazon) will work with a phone with app iOBD2 and provide that real time pressure value (but not sure).

Any thoughts? Buy the ELM or just put a new pressure sensor on it and see if that fixes it? It seems I can feel it over the LHS frame rail and I think it just pulls out.

Thanks in advance!

Original gas cap ?

If so I'd start by replacing it and clearing the codes

Yes. I think I am going to replace it next.

Also clean the top of the filler. Could be build up on top of it causing it to not seal well.

All, just wanted to post an update in case someone else is trying to troubleshoot GM evap problems. I purchased the "Torque Pro" app for Android and this Bluetooth OBD adapter:

https://www.amazon.com/BAFX-Products-Wireless-Bluetooth-Diagnostic/dp/B005NLQAHS

The canned GM PIDs in Torque Pro would not read the fuel tank pressure sensor on my 2007 Silverado. It would on my 2011 Tahoe. So, I found the below PID and added it and I can now read the fuel tank pressure sensor voltage. Turns out the sensor was railed at 4.8V (supposed to be reading about 1.5V at atmospheric pressure), so it was the cause of the P0446 code.

This is the PID I added to Torque Pro. This is by far the best OBD2 app because you can add custom PIDs if you can find them on the web like I did this one:

"Fuel Tank Pressure -F", "FuelTnkPr", "22114E", "A/51", 0, 5, "Volts", "Auto"

Did you DIY?

It looks like a bear in photos.

I haven't actually replaced it yet. Ordered it earlier in the week. I can actually get to it by reaching up and over the frame rail. Plan to try and use a blow gun to get the dirt/dust off and then unlug it and pull it out and replace. I think it should be doable. Just won't be able to see what I am doing.....