My CEL just lit up on my 04 Silvy. I took it to a parts place for a scan, and they said "small emissions leak" but didn't tell me a specific code. CEL has never been on before. I checked the engine and all the hoses and can't find anything cracked or loose. No obvious exhaust system leaks. I traced all the hoses back to the gas tank, but I can't see the lines on top of the tank. Truck still runs great, new 5.3L has 6900 miles on it.

What do I need to check?

 

I don't know if that year does it but vehicles do it for a loose gas cap.

 

Check rubber hoses from the filler to the tank for cracks and gas cap. After that time to smoke test the evap system with UV smoke. That codes bane of mechanics existence who live where you gotta deal with that useless crap.

 

Gas cap would be more first look. Either loose or you need a new one.