Hello I'm new to the forum. I have read several threads here but now I have a question of my own. I have a 2003 Monte Carlo ss and check engine light on. Went to AutoZone and was told bad gas cap. I replaced gas cap and week later check engine light still on. Anyone have any ideas?

 

Check vac lines underhood

 

Welcome to the forum...

It could also be a bad cracked leaking return line to fuel tank.
also check under the flap in the trunk and check the lines connected to the fuel pump inspect them for signs of rubbing or crack/dry rot etc...

Do you ever smell gas in or around the vehicle?

I too would also check all the vacuum lines under the hood...