2004 chevrolet cavalier engine light on. OBD2 has

also check make sure your fuel filler cap is on. a loose or defective cap will set the light. The filler neck themselves also rust out. If you check these few things and it still stays on afet a few days you mite have to take it to a shop they seal the system with a scan tool and fill the system with a nitrogen smoke so they can see where the smoke is leaking from.

There's a thing called a evap solenoid or purge solenoid under the rear of the car. Right behind the passenger rear tire. It has a hose coming into it and a wiring harness about 8 inches long that has only two wires. One is white I don't recall what color the other was seems like it might have been pink. But that is what the problem with mine was. You have to get one from a junk yard or a dealership. It may also be the actual wiring harness coming out of the solenoid sometime they break.