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

We tried this the other night to see what the problem was with my oil pressure gage. We know that the gage works find so the problems has to be something internal.