Question 1

What does Jeep fault code 11 mean?

Accepted Answer

11: Engine cranks but won't start; Sudden stalling when hitting bumps; No spark. Fix: No Crank Reference Signal.

LOGIC: The PCM is not seeing a signal from the Crankshaft Position Sensor (CPS) while the engine is turning.

ACTION: 1. Check the CPS connector at the back of the engine (near the firewall). 
2. Inspect the CPS wiring for heat damage from the exhaust manifold.

FIX: Replace the Crankshaft Position Sensor.

Question 2

What does Jeep fault code 12 mean?

Accepted Answer

12: No driveability symptoms; Usually appears after battery work. Fix: Battery Disconnect.

LOGIC: The PCM has lost constant 12V power within the last 50 key-on cycles.

ACTION: 1. Ensure battery terminals are tight and free of corrosion. 
2. Verify the "Memory Power" fuse is not blown.

FIX: No repair needed if battery was recently disconnected; otherwise, check for loose battery cables.

Question 3

What does Jeep fault code 14 mean?

Accepted Answer

14: Rough idle; Engine "running rich" (smell of unburned gas); Poor throttle response. Fix: MAP Sensor Voltage Too High/Low.

LOGIC: The Manifold Absolute Pressure sensor signal is outside the expected 0.5V - 4.5V range.

ACTION: 1. Check the vacuum line between the throttle body and the MAP sensor. 
2. Verify 5V reference at the sensor.

FIX: Replace the MAP Sensor or repair the cracked vacuum line.

Question 4

What does Jeep fault code 21 mean?

Accepted Answer

21: Poor fuel economy; High idle; Engine stays in "Open Loop" mode. Fix: Oxygen (O2) Sensor Signal Stale.

LOGIC: The O2 sensor voltage is not "switching" fast enough, or it's stuck at center voltage (0.5V).

ACTION: 1. Inspect O2 sensor wiring (it often melts against the exhaust pipe). 
2. Check the O2 sensor heater fuse in the Power Distribution Center.

FIX: Replace the Oxygen Sensor or repair melted wiring.

Question 5

What does Jeep fault code 24 mean?

Accepted Answer

24: High idle; Transmission shifting issues (XJ Auto); Engine won't return to idle quickly. Fix: Throttle Position Sensor (TPS) Voltage High/Low.

LOGIC: The TPS signal is out of range or has "dead spots" in the internal resistor.

ACTION: 1. Test the sensor with an analog voltmeter; look for a smooth needle sweep as you open the throttle. 
2. Check for 5V reference.

FIX: Replace the Throttle Position Sensor.

Question 6

What does Jeep fault code 27 mean?

Accepted Answer

27: Misfire under load; Engine "stumbling" at highway speeds. Fix: Injector Control Circuit.

LOGIC: The PCM is not seeing the expected inductive "kick" from one or more fuel injectors.

ACTION: 1. Use a "Noid Light" to check for pulse at each injector. 
2. Measure injector resistance (should be approx. 12-14 Ohms).

FIX: Replace the faulty Fuel Injector or repair the injector harness.

Question 7

What does Jeep fault code 33 mean?

Accepted Answer

33: No driveability symptoms unless A/C is in use. Fix: A/C Clutch Relay Circuit.

LOGIC: The PCM is commanding the A/C clutch on, but detecting no current flow (Open or Short).

ACTION: 1. Check the A/C clutch relay in the under-hood fuse box. 
2. Check the A/C fuse.

FIX: Replace the A/C Clutch Relay or repair the compressor clutch coil.

Question 8

What does Jeep fault code 42 mean?

Accepted Answer

42: Engine won't start or dies immediately; No fuel pump prime sound. Fix: Auto Shutdown (ASD) Relay Circuit.

LOGIC: The PCM has lost control of the ASD relay, which provides power to the fuel pump and ignition coil.

ACTION: 1. Swap the ASD relay with the Horn relay (they are usually identical) to test. 
2. Check for a short in the fuel pump wiring.

FIX: Replace the ASD Relay or repair the fuel pump harness.

Question 9

What does Jeep fault code 51 mean?

Accepted Answer

51: Lean misfire; Hissing sound; Hesitation on acceleration. Fix: Oxygen Sensor Indicates Lean.

LOGIC: The O2 sensor is stuck "low," and the PCM has maxed out its fuel compensation.

ACTION: 1. Check for intake manifold vacuum leaks. 
2. Inspect for exhaust leaks at the "cracked" manifold (very common on 4.0L).

FIX: Replace the cracked Exhaust Manifold; repair vacuum leaks.

Question 10

What does Jeep fault code 55 mean?

Accepted Answer

55: No symptoms. Fix: End of Message.

LOGIC: All stored codes have been displayed.

ACTION: None.

FIX: None.

Code	System	Symptom	Full fix
11	—	Engine cranks but won't start; Sudden stalling when hitting bumps; No spark.	View fix
12	—	No driveability symptoms; Usually appears after battery work.	View fix
14	—	Rough idle; Engine "running rich" (smell of unburned gas); Poor throttle response.	View fix
21	—	Poor fuel economy; High idle; Engine stays in "Open Loop" mode.	View fix
24	—	High idle; Transmission shifting issues (XJ Auto); Engine won't return to idle quickly.	View fix
27	—	Misfire under load; Engine "stumbling" at highway speeds.	View fix
33	—	No driveability symptoms unless A/C is in use.	View fix
42	—	Engine won't start or dies immediately; No fuel pump prime sound.	View fix
51	—	Lean misfire; Hissing sound; Hesitation on acceleration.	View fix
55	—	No symptoms.	View fix
B1239	Climate Control (HVAC)	Air Trapped at Floor Vents	View fix

Jeep Fault Codes

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