I had the same problem. My mechanic said it has to do mostly with the emissions system however it could be something else as well. I replaced the emissions pump in the rear whell well for $500 and the light came back on 8 miles from the shop. It goes back in on Monday.
I have read on this board that Autozone will read the error codes for you for free. I would start there. If there isn't an Autozone in your area call around to the other shops and ask if they have a OBD 2 scanner you could use. There is a master list of the meanings of the numbers it gives you on this board too. Good Luck!
