My check Engine light has been coming on and off for 6 months now, mostly on, I have it connected 3 times to the computer at the garage and I have gotten 2 error codes (can't remember what the actual numbers were) the first and the third were throttle sensor error
and the second was transmission not shifting from second gear (that one was recently only 3 days ago. had drastic loss of power, when from 60 kph to 20 kph on a hill....not fun and it was late at night when this happened too...)
I had the same issue, and like the knucklehead I am, I ignored it after it went off. Finally I got a letter from the state telling me I had to pass an emissions test before I could reregister the car. I took it to Pep Boys, they got three codes and told me to go to the dealer. I want to the state inspection depot and they got three totally different codes. I finally went to the Chevy dealer and they got five completely different codes. Final verdict: replaced ALL the sensors and spark plugs. Total plus labor :$1,033. Maybe it would have been lower if I went earlier. Moral of the story, don't ignore your CEL.