92 4.0 XJ check engine light????

My wifes 92 4.0 XJ after runnng for a few minutes has "check engine" light come on. What could this be from and how do I eve n start to diagnose?? Starts & runs great. Thanks, Heinz

First off check the gas tank cap, loose ones will cause it to come on. Next look for vacum leaks and disconnected hoses. How long does it stay on for or does it stay on as long as it's running ?? How many miles since its last tune up ?? Could also be a bad O2 sensor or clogged cat.
Run some Chevron Techron thru the fuel system first, clean the throttle body out. Quick check would be to take it to Auto Zone, some here have said they will hookup a diagnostic reader and pull the code. I also think you can use the ignition switch to get the dash leds to flash the trouble code. I have a 98 and they will not do this.

On a 92, you cycle the ignition switch ON-OFF-ON-OFF-ON (i.e. turn it on 3 times in quick succession). The check engine light will then flash number codes.

You can find out the meaning of the codes on this site: http://www.allpar.com/fix/codes.html

Gas tank cap good, no loose vacuum lines, just put new cap & rotor on a few weeks ago, plugs good, just started a few days ago wife says, will try cleaning TB, and almost always every oil change run B12 Cheemtool thru tank(1 can). Will try to find AutoZone here , that would be great if they could pinpoint it for me! If bad 02 , then wouldn't it get bad mileage/rough start? For bad cat, I would think bad response/acceleration etc., it runs perfect. For igniton code LEDS, what do I look for?
Thanks, Heinz

What millage are you at? I also have a 92 and around 150000 miles my light came on with no problem what so ever. I can't rember what I did to get it to go off, but I was told by many mechanics that a lot of these lights are programed to come every so many thousand miles just so the owner will come in to the shop to spend some cash. Nothing was ever wrong with mine. I think I even pulled the fuse of somnething.

You don't mean "Engine Maintenance" light do you? Mine came on in other Jeep at exactly 120K. No problems. My wifes Jeep has "CHeck Engine" light on for a few days and runs perfect though, Thanks, Heinz

On my '91 the Engine Maintenance light came on at 85000. Chiltons said that this was intentional to indicate that the Oxygen Sensor required replacing. That was an easy job. Getting someone to reset the computer to turn the light off was a BEAR!.

Now, the Engine Maintenance light is back on at just over 120000. Can't find a reason yet.