I seem to recall reading somewhere, that on some Jeeps, you can close all the doors, have the key in the off positions, and (without starting it) turn the key to on, off, on, off, on, within 5 seconds, and it will show the code on the odometer?
er....as per
this link, it depends on if you have the sentry key system:
For a 1998 - Present TJ
in 1998, Chrysler upgraded the Jeep's internals to include their revised system used across many Chrysler fuel-injection models. This newer system still provides for reading the codes yourself. But how you do it and actual codes themselves are different from the 1997 and earlier models.
The method you use to view the engine diagnostics is now different. Starting in 1998, the method of repeatedly switching on/off of the ignition is now used for programming the new Chrysler Sentry Key system. The Sentry Key was introduced in 1998 as an optional feature and by 2000 was standard on all TJs. Some 1998 models without the Sentry Key system may still use the older procedure. The way to tell if your Jeep has the Sentry Key system is if the plastic head on your keys are gray--you do. Black headed keys mean you don't have the Sentry Key system.
So on TJs with the newer diagnostic system, to see the codes you put the key into the ignition, push and hold down the odometer reset button, turn the key to RUN, then release the odometer reset button. When you do this you are rewarded with dancing number displays, the firmware version number, any error codes, then a diagnostic test of all the dash indicators and gauges. The gauges will, one by one, go through a calibration--the needles moving and stopping at major increments so you can check their alignment with the dial markings.
Output from the newer onboard diagnostics is easier to read now, as you get full 4-digits codes for much greater detail.
So hold down your tripometer reset button and turn the vehicle to on/run (but don't start it), then release the tripometer reset button, and it should display a bunch of stuff.
I'm gonna try this when I get home, see how the gauge calibration stuff works, sounds nifty :p