I have a 2012 Chevy Silverado 1500LT 4x4.

Exactly what codes are you getting? It sounds like you have a code for the EVAP system. To properly find the leak, which may not be visible to the naked eye, you need to have the system smoke tested. Harmless smoke is forced into the system through the gas tank by a smoke machine. The mechanic then carefully checks the entire system to see where smoke is coming from. When he finds the smoke, he's found the leak. Hope that helps! Jim

EVAP leaks are the pits. I had a truck that kept coming back with a leak code, I smoke tested the system, didn't find anything, cleared the code and drove it, code did not return during the test drive so I gave it back, he came back two days later with the same problem, so again another smoke test, still no leak, so I decided maybe I would drag out my old fuel system pressurizer unit the one that uses CO2, so I applied pressure to the system and watched the gauge, it showed a slow leak, so I got my listening hose out and started going all over the fuel system, I finally found a small hissing noise at the fuel filler around the cap, now this cap was new so I ruled the cap was not the problem, I looked at the filler inlet itself and I found a series of small nicks in the spout right where the cap was suppose to seal the opening, later he told me he keeps banging the fuel nozzle into the fuel pipe when filling up, got a new filler piper and installed it, did another pressure test, no sounds, did a smoke test just to be sure, no leaks, cleared the code and did a road test no check engine light so i gave it back to him. He returned a month later for a Power window motor not working and still no check engine light. HTH