Interesting problem with Gas cap message.

My CEL has been on for a while now and I keep getting the "message" so I replaced the gas cap. Light went out for a few days but came back on. Now when I fill the truck light goes out, after using about 1/4 tank message and light come back on. Any suggestions on what this may be pointing to? Like maybe the pressure sensor on the tank or the purge valve or even the canister? Not sure what a diagnosis at the dealer would cost but running a little low on funds and if it's something I can fix myself I will.

What model/year/engine? If you haven't retrieved any codes from pcm yet, that's the first place to start. If you don't have a code reader, you may be able to find an auto parts store that does. If/when you get a code, remember that the code isn't telling you to replace a part, it's telling you what system is having problems.

2004 Silverado 5.3. If I remember right I believe it was small leak detected. I don't remember the exact code.