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Diagnostic Trouble Codes

When a late-model, computerized Jeep is having engine problems, one of the ways it communicates these problems back to the driver is to turn on the “check engine” light on the dash. One idiot light isn’t very descriptive, so Jeep also provided a method to query the computer for all codes that have been set. Some of these codes are important enough to turn on the check engine light, while others are minor enough not to require immediate attention and therefore will not turn on the dashboard light by themselves.

To query and OBD-I-era computer for these codes, put the key in the ignition and turn the key to run (not start), then off, then run, then off, then run (that’s 3 runs and 2 offs). Leave the key in the run position. The check engine light will turn on for a few seconds, then turn off, then (sometimes after 20-30 seconds) start flashing. The flashes are counting out 2-digit trouble codes. Fast flashes count up each digit, with a longer pause between digits. On TJ’s with a digital odometer, the code numbers are printed on the odometer in addition to flashing the check engine light. 55 indicates the end of all codes. Once you’ve got your code list, consult the table below to see what they mean.

OBD-II information is more complex, so the codes for 1998 and newer Jeeps can’t be displayed by flashing the check engine light. However, those vehicles which have digital odometers will display the codes on the odometer. I’ve verified this personally on a 1999 WJ. For those vehicles without digital odometers, or for anybody who wants an English description rather than just a numeric code, an OBD-II code scanner ($20-300) can be plugged in under the dash. OBD-II codes can be looked up online¬†here.

These codes were taken from the ’95 XJ/YJ factory service manual, and probably apply to most other OBD-I-era Jeeps.

CodeDescription
11*No crank reference signal detected during engine cranking.
12*Direct battery input to PCM was disconnected within the last 50 key-on cycles.
13**No difference recognized between the engine MAP reading and the barometric (atmospheric) pressure reading at start-up
14**MAP sensor input below minimum acceptable voltage.
or
MAP sensor input above maximum acceptable voltage.
15**No vehicle distance (speed) sensor signal detected during road load conditions.
17*Engine coolant temperature remains below normal operating temperatures during vehicle travel (thermostat).
21**Neither rich nor lean condition detected from the oxygen sensor input.
or
Oxygen sensor input voltage maintained above the normal operating range.
22**Engine coolant temperature sensor input above maximum acceptable voltage.
or
Engine coolant temperature sensor input below minimum acceptable voltage.
23**Intake manifold air temperature sensor input above maximum acceptable voltage.
or
Intake manifold air temperature sensor input below minimum acceptable voltage.
24**Throttle position sensor input above maximum acceptable voltage.
or
Throttle position sensor input below minimum acceptable voltage.
25**A shorted condition detected in one or more of the idle air control motor circuits.
27*Injector #1 output driver does not respond properly to the control signal.
or
Injector #2 output driver does not respond properly to the control signal.
or
Injector #3 output driver does not respond properly to the control signal.
or
Injector #4 output driver does not respond properly to the control signal.
or
Injector #5 output driver does not respond properly to the control signal.
or
Injector #6 output driver does not respond properly to the control signal.
33*An open or shorted condition detected in the A/C clutch relay circuit.
34*An open or shorted condition detected in the speed control vacuum or vent solenoid circuits.
or
Speed control switch input below the minimum acceptable voltage.
or
Speed control switch input above the maximum acceptable voltage.
35*An open or shorted condition detected in the radiator fan relay circuit.
41*An open or shorted condition detected in the generator field control circuit.
42*An open or shorted condition detected in the auto shutdown relay circuit.
44*An open or shorted condition exists in the engine coolant temperature sensor circuit or a problem exists in the PCM's battery temperature voltage circuit.
46**Battery voltage sense input above target charging voltage during engine operation.
47**Battery voltage sense input below target charging voltage during engine operation. Also, no significant change detected in battery voltage during active test of generator output.
51**Oxygen sensor signal input indicates lean air/fuel ratio condition during engine operation.
52**Oxygen sensor signal input indicates rich air/fuel ratio condition during engine operation.
53*PCM internal fault condition detected.
54*No fuel sync (camshaft signal) detected during engine cranking.
55*Completion of diagnostic trouble code display on the malfunction indicator lamp (check engine lamp).
62*Unsuccessful attempt to update SRI (service reminder indicator) miles in the PCM EEPROM.
63*Unsuccessful attempt to write to an EEPROM location by the PCM.

* – Check engine lamp will not illuminate at all times if this code was recorded. Cycle ignition key as described and observe code flashed by check engine lamp.

** – Check engine lamp will illuminate during engine operation if this code was recorded.

PCM = Powertrain Control Module (the engine computer)
MAP = Manifold Absolute Pressure (the sensor that measures how hard the engine is sucking air through the intake manifold
EEPROM = Electrically Erasable Programmable Read Only Memory (a type of computer chip used inside the PCM)

[Last updated 1 Sep 2008]

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2 thoughts on “Diagnostic Trouble Codes

    1. Obi-Wan Post author

      Interesting. The ’95 factory service manual doesn’t list 76 as a possible code. However, google tells me that you’re not the first person to encounter this:

      https://www.jeepforum.com/forum/f12/trouble-code-76-a-1443074/

      Generically, 76 indicates an open or short in the fuel pump relay ballast resistor. However, a ’94 YJ doesn’t have a ballast resistor, so it shouldn’t ever throw that code. Does your Jeep still have the original engine, or was it replaced somewhere along the way with an older or rebuilt engine? That might explain why you’re getting that code.

      If you do have a ballast resistor, it’ll be located in the engine bay by the brake master cylinder. It’s a white ceramic box roughly 3/4 x 3″ in size with one wire sticking out each end. Here’s a video that shows how to test & replace it:

      https://www.youtube.com/watch?v=0M9piFUSDwo

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