I have been having a recurring CEL due to a Evap error code. I just wanted to post this so if anyone else experiences this it might save you some time troubleshooting your truck. Keep in mind this is all from the same Evap error code every time. Also keep in mind this truck only has 32,000 miles and is well maintained.

I first bought a new OEM gas cap from the dealer- Did not fix CEL returned after about 7-10 days
Took to dealer, they replaced a fuel vent valve (or something similar)-Did not fix, CEL returned after about 7-10 days
Returned to dealer, they replaced a fuel sensor switch(something to that effect)-Did not fix, CEL returned after 7-10 days
Returned to dealer, they said it was the gas cap. I explained I had already bought a OEM cap from them, they still replaced it with another one.-Did not fix, CEL returned about 7-10 days

Returned to dealer about 2-3 weeks after CEL had come on and they said they couldn't get a reading because it had been on for so long. They cleared the code and said come back as soon as it comes back on.

Went to dealer within hours of CEL coming on, they tried to say it was the gas cap, again... cleared code and sent me home. 2 days later CEL came back on.
I emailed Chevy service directly and got a district service manager involved. I took my truck in and this time they decided to pull the gas tank, said there was something partially clogging the sending unit. They have kept it overnight and are supposed to be testing it this morning. Hopefully this will be the last time I have to deal with this issue. Hopefully this will help you if you start to experience the same problems.

Overall if I hadn't of complained to Chevy directly I don't think this particular dealership would have ever put any effort into fixing my truck. When they finally decided that they would have to do some real work on it, (not just run the code, and clear it) they didn't want to give me a loaner car, and actually complained about not having one left if they gave me one. I explained to them that the district manager had said they would give me one, they finally let me take it.

Lastly, the district service manager told me that he couldn't guarantee that the technicians that were working on my truck were actually certified techs... I thought that was a prerequisite to work at a dealership?