I have a 96 xj and my check engine light is currently on it has been for over a year. I have had no problems from it the jeep runs fine but i need to get it inspected. but when i plug it in to check the codes nothing shows up, i have done this on a few separate machines to make sure it wasn't the machine error. The only way to reset the light is by disconnecting the battery but the light just comes back on after a few minutes. Any ideas of what could cause the problem.

 

Most of the time when you check the codes and nothing comes up it's a more complicate code than what the system you are using can read. I've heard these called "hard codes" even the dmv analyzer I use at the shop I work at has codes like that that it cannot diagnose unfortunately. The other problem with disconnecting the battery is that then you gotta drive it around a lot to get passed the "not ready rejection"

 

In New Mexico you will still pass with two not ready codes

 

Yes that is true as well for vehicles under 2000 but if he is resetting his light and it is immediately coming back on then he has a bigger problem then just the not ready rejection. Plus it will still take some driving for the rest of the codes to reset

 

Try this.
http://go.jeep-xj.info/HowtoECUcodes.htm
and see if the Jeep can diagnosis itself. If all else fails take it in and see what the the emissions shop's scanner says.