Yes the gas cap causing the check engine light on is a common problem. It has a sensor that monitors the pressure or something and is easily fixed just take off the cap make sure you place it securely in the fitting and tighten it. Light should go out after running it around town for a little while. If you have a after market cap I'm not so sure ther is anything you can do but make sure it is specifically for your model and and year. I got a locking one from advance wich they told me would work but it didn't, so double check them. There should be no threads sticking out of the cap fitting. I believe there is a way also to check the engine codes on 99 and newer jeeps by pressing and holding the odometer reset button in while you turn your key to the on position (don't start it). should give you a software version number,followed by engine codes if something wrong, then a gauge check will be performed. After that you can look on the forum for the codes and check it yourself.:o Don't be tricked into paying money to have a mechanic do a diagnostic for you
. My local Jeep stealership charges $90. just to do this. If you can't get this feature to work just take it to advance auto or auto zone and they will to a diagnostic for free.