Check Engine light keeps coming on

[HummerJim]

Autozone will read your codes for free. I had a similar problem with my old H3 and it turned out to be the pre cat O2 sensor. There is a history and some old threads about the gas caps going bad and giving the check engine light. I'll see if I can find you the link. I have the GT1 diagnostic software for my laptop and the Hummer plug in and I love it. You can use it on all domestic cars and BMWs, Porsches and Mercedes if you get the proper cables and plugs.