There is a way to check the codes by turning the ignition on and off and counting the flashes. Here is a thread I copied awhile back:
Self-Diagnostic Codes: Retrieval and Mysterious Meaning
Applies to years 91 and newer YJ
The best way to tell if your Jeep is storing a code is if the "CHECK ENGINE" light comes on. (Or "POWER LOSS" as some Haynes manuals for YJs will state.)
Your Jeep's Keys.
A pen/pencil (to record codes with).
Paper (to record codes).
To retrieve codes (even if the light isn't on but you think something is wrong) simply:
1) Set the parking brake and put your transmission in NEUTRAL (MANUAL) or PARK (AUTOMATIC).
2) *According to Haynes Maual*: Run the engine to 2,500RPMs in the appropriate above gear. Cycle the A/C (if you still have one) briefly, then turn it off. [Automatic Only] With your foot on the brake, shift into each gear position until you reach L (Low/First) then begin shifting back until you reach park.
3) Now on to actually retrieving your codes. Turn your ignition key to "ON", "OFF", "ON", "OFF", and back to "ON" and leave it there.
4) Watch the ENGINE LIGHT (or POWER LOSS light). It will blink to indicate numbers. It will 'blink to indicate the first number of the digit then pause and blink to indicate the second digit of the number'.
4 Example) 23 MAT Sensor Circuit: Check engine light would blink twice for the 2, then pause (About 2 seconds) and blink 3 times for the 3. Put the two together and you have 23.
NOTE: The pause between DIGITS OF A CODE is Approximately 2 seconds.
NOTE: The pause between CODES is Approximately 4-5 seconds. (IF Multiple Codes are being reported)
NOTE: When you receive CODE 55 it means the Diagnosis is done. No more codes will be generated until you restart the process.
Haynes Note: "Certain Criteria must be met for a fault code to be entered into the computer memory. The criteria may be a specific range of engine RPM, engine temp. or input voltage to the computer. It is possible a fault code will not be entered into memory despite a malfunction".
SELF-DIAGNOSTIC CODES (1991 and later models):
CODE: Probable Cause:
13 Manifold Absolute Pressure (MAP) Sensor Vacuum
14 Manifold Absolute Pressure (MAP) Sensor Electrical
15 Distance Sensor or Circuit
17 Engine Running Too Cool
21 Oxygen Sensor or Circuit
22 Coolant Temperature Sensor or Circuit
23 MAT Sensor or Circuit
24 Throttle Position Sensor (TPS) sensor or Circuit
25 Automatic Idle Speed (AIS) Circuit
27 Fuel Injector Control
33 Air Conditioning Clutch Relay
41 Alternator Field
42 Automatic Shutdown Relay
44 Battery Temperature Sensor
46 Battery Over Voltage
47 Battery Under Voltage
51 Oxygen Sensor - lean condition indicated
52 Oxygen Sensor - rich condition indicated
53 Internal Engine Controller Fault
54 Distributor Sync Pickup
55 End of Code Output
62 Emissions Maintenance Reminder (EMR) mileage accumulator
63 Controller Failure EEPROM write denied
*Mostly Thanks To The Haynes Manual*