check engine light was off after repairs back on again why??

engine a 5.2L it says had evap codes stored in computer, gas cap loose no other problems found light was out...only a few days later light on replaced gas cap like suggested light still on...any suggestions thanks I don't have code to give you I took my van to another repair shop Oct1st it is still there, they r watching battery dies after 3 days said the icon on steering wheel that tells u drive park reverse etc. stays on why? this is draining battery let them keep van to continue checking causes i don,t have codes to give u a good mechanic is hard to find and an honest one these days years ago this wasn't an issue like today its a shame.