When I got the car last year, the check engine light was on. I read/reset the CEL at the time to see how long it would take to come on again... and it didn't.
Until this weekend! When (of course) I was getting ready to take the car in for its e-test.
It came on for
p0455 -- EVAP system major leak.
This is the infamous "your gas cap is loose/missing/defective" CEL, so I swapped caps with the known-good one from the Flea and reset the CEL. Hopefully that's it, and not an actual problem with the EVAP plumbing/components.
Now I just have to do a few more drive cycles to see if the light comes on again.
The guys at the e-test facility said GM OBD-II cars are the worst for getting the system montors to return "READY" status (required to pass e-test). They said GM's seem to take a lot more distance/cycles than other makes to self-test its various systems.