2002 Chevy Silverrado ext cab service engine light

I have a 2002 Chevy Silverrado ext cab with a 5.3 liter engine.
I am having the service engine light come on.I had it to a dealer and they replaced I think he said the purge solenoid on top of the engine.It was an emmision code of some kind.After he replaced the purge solenoid it was fine for about a month.Then service engine light came back on.I took it back and they could not find any problem or why the service engine light came back on.
I replaced the gas cap and filled up the gas tank and the next day the light went off.However after the tank dropped about a quarter of a tank the light came back on.I filled the tank again and the next day the light went of again.
This happens evry time I fill up the next day the service engine light goes off.
He said the code told him the solenoid was bad but he checked and said it was ok.
Any ideas would be appreciated.