If your TJ has a check engine light illuminated, below is the procedure for retrieving the diagnostic trouble code(s):
Turn the ignition key until the digital odometer displays, then turn it back to off. Repeat three times in succession. (ON/OFF/ON/OFF/ON)
Count the number of times the check engine light on the dash flashes on and off. The number of flashes represents the trouble code. There is a slight pause between the flashes representing the first and second digits of the code. Longer pauses separate different trouble codes.
For example, light flashes 5 times, then a short pause, then flashes 5 more times. This is code 55.
Diagnostic trouble codes will also be shown in the odometer display.