Decoding Your 1997 Jeep TJ Check Engine Light: OBD2 and Diagnostic Codes

Navigating the diagnostic system of a 1997 Jeep TJ can be a unique experience, especially when it comes to understanding the check engine light and error codes. If you’re a Jeep TJ owner, particularly of the 1997 model, you might have encountered some confusion regarding OBD1 and OBD2 compatibility. The truth is, the 1997 Jeep TJ occupies a bit of a special spot in automotive diagnostics. It’s not strictly OBD1 nor is it fully OBD2 in the way later models are. This article will guide you through understanding your 1997 Jeep TJ’s diagnostic system, reading error codes without expensive scan tools, and what those codes mean.

Understanding the 1997 Jeep TJ Diagnostic System

The original article correctly points out that standard OBD2 code readers might not be fully compatible with the 1997 Jeep TJ. This is because the 1997 TJ utilizes a hybrid system that predates the full standardization of OBD2. While it incorporates some aspects of OBD2, it doesn’t fully adhere to the protocols that later OBD2 scanners expect. This is why you might see warnings on some generic code readers stating incompatibility with 1997 Jeep TJs.

For dealerships and professional mechanics back in 1997, the go-to tool was the DRB scan tool – a specialized and costly device. However, thankfully for the DIY enthusiast and budget-conscious Jeep owner, there’s a built-in method to retrieve diagnostic trouble codes directly from your Jeep TJ without needing to purchase expensive equipment.

The Key Cycling Trick: Reading Codes Without a Scanner

The method described in the original article, often referred to as the “key cycling method,” is the secret to accessing your 1997 Jeep TJ’s diagnostic codes. It’s an ingenious way Chrysler engineers provided a diagnostic capability without relying solely on external scan tools for basic troubleshooting. Here’s how to do it:

Ensure the ignition is OFF.
Turn the ignition key to the ON position (without starting the engine).
Turn the ignition key back to the OFF position.
Turn the ignition key to the ON position again.
Turn the ignition key back to the OFF position.
Finally, turn the ignition key to the ON position.

Do all of these steps within 5 seconds. If successful, and if there are any stored diagnostic trouble codes, the “Check Engine” light (or Malfunction Indicator Lamp – MIL) on your dashboard will begin to flash two-digit codes.

Reading the Codes:

The codes are displayed as two-digit numbers. For example, you might see a flash, flash, pause, flash, flash, flash, pause, and so on. This would be interpreted as code 12 (flash, flash = 1) and code 3 (flash, flash, flash = 3). The codes will repeat, and code 55 signals the end of the code display sequence.

Important Notes:

If no codes are stored, or if the system doesn’t detect any faults, you will likely see code 55 displayed immediately, indicating “End of Test.”
Some codes will illuminate the check engine light, while others might be stored in the system without turning on the light.
If you have a check engine light illuminated and the key cycling method doesn’t display any codes (other than perhaps code 55), it might indicate a problem with the PCM itself or the diagnostic display system, and in such cases, a DRB scan tool or professional diagnosis might be necessary.

1997 Jeep TJ Error Codes Explained

Here’s a breakdown of the two-digit error codes for the 1997 Jeep TJ, as provided in the original article, with some added context for better understanding:

Error Code Descriptions:

11* No crank reference signal detected during engine cranking. / 11** Intermittent loss of either camshaft or crankshaft position sensor. CKP sensor target windows have too much variation. – This often indicates a problem with the Crankshaft Position Sensor (CKP) or Camshaft Position Sensor (CMP). These sensors are crucial for engine timing. A faulty sensor can prevent the engine from starting or cause it to run poorly.
12* Direct battery input to PCM was disconnected within the last 50 Key-on cycles. – This is a general code that often appears after battery disconnection and isn’t usually a cause for concern unless it persists.
13** No difference recognized between the engine MAP reading and the barometric (atmosphere) pressure reading from start-up. – Indicates a potential issue with the Manifold Absolute Pressure (MAP) sensor. The PCM expects the MAP sensor to read atmospheric pressure at startup.
14** MAP sensor input above or below acceptable voltage. 5 volt output to MAP sensor open. – Another MAP sensor related code, suggesting a voltage issue – could be wiring, sensor malfunction, or PCM problem.
15** No vehicle speed sensor signal detected during road load conditions. – Problem with the Vehicle Speed Sensor (VSS). This sensor is used for various functions including speedometer operation and transmission control.
17** Engine did not reach operating temperature within acceptable limits. Engine does not reach 20º F. within 5 minutes with a vehicle speed signal. – Indicates a problem with the engine warming up. Could be a thermostat issue or problem with the Engine Coolant Temperature (ECT) sensor.
21** Upstream oxygen sensor response slower than minimum required switching frequency. Upstream oxygen sensor heating element circuit malfunction. Downstream oxygen sensor heating element circuit malfunction. Downstream oxygen sensor input voltage maintained above the normal operating range. Oxygen sensor voltage too low, tested after cold start. (Upstream or Downstream) Left oxygen sensor input voltage maintained above the normal operating temperature. – A range of oxygen sensor (O2 sensor) related issues. These sensors are vital for fuel mixture control and catalytic converter efficiency. Codes can indicate sensor failure, heater circuit problems, or rich/lean conditions.
22** Engine coolant temperature sensor above or below acceptable voltage. – Problem with the Engine Coolant Temperature (ECT) sensor. This sensor is crucial for engine temperature monitoring and fuel mixture adjustments.
23** Intake air temperature sensor input above or below acceptable voltage. – Issue with the Intake Air Temperature (IAT) sensor. This sensor measures the temperature of the air entering the engine.
24** Throttle position sensor input above or below acceptable voltage. TPS signal does not correlate to MAP sensor. – Problem with the Throttle Position Sensor (TPS). This sensor tells the PCM the throttle valve position.
25** A shorted or open condition detected in one or more of the idle air control motor circuits. Actual idle speed does not equal target idle speed. – Issue with the Idle Air Control (IAC) valve or circuit. The IAC valve controls idle speed.
27** Injector #3, and/or 4, and/or 5, and/or 6 output driver does not respond properly to the control signal. – Indicates a problem with the fuel injector circuits for injectors 3, 4, 5, or 6.
31** An open or shorted condition detected in the A/C clutch relay circuit. Insufficient or excessive vapor flow detected during evaporative emission system operation. – Relates to the Air Conditioning (A/C) clutch relay or Evaporative Emission (EVAP) system.
33* An open or shorted condition detected in the duty cycle purge solenoid circuit. – Problem with the EVAP purge solenoid circuit. Part of the emissions control system.
34* An open or shorted condition detected in the Speed Control vacuum or vent solenoid circuits. Speed control switch input below the minimum acceptable voltage. – Issue with the speed control (cruise control) system.
37** Relationship between engine speed and vehicle speed indicates no torque converter clutch engagement. An open or shorted condition detected in the torque converter part throttle unlock solenoid control circuit. Incorrect input state detected for the Park/Neutral switch. Auto only. – Transmission related code, specifically Torque Converter Clutch (TCC) engagement or Park/Neutral switch (automatic transmissions only).
41*** An open or shorted condition in the generator field control circuit. – Charging system problem, related to the alternator/generator field control circuit.
42* An open or shorted condition detected in the auto shutdown relay circuit. An open condition detected in the ASD relay output circuit. An open or shorted condition detected in the fuel pump relay control circuit. An open circuit between PCM and fuel gauge sending unit. Circuit shorted to voltage between PCM and fuel gauge sending unit. No movement of fuel level sender detected. – A broad code indicating issues with the Auto Shutdown (ASD) relay, fuel pump relay, or fuel gauge sending unit circuit. The ASD relay is critical for engine operation.
43* Peak primary circuit current not achieved with maximum dwell time / 43** Misfire detected in one or more cylinders 1 thru 6. (4 and 6 cyls.) – Ignition system issue. Code 43 can indicate an ignition coil problem or misfire.
44** Battery temperature sensor in voltage above or below acceptable range. – Problem with the Battery Temperature Sensor (if equipped).
46*** Battery voltage sense input above target charging voltage during engine operation. – Overcharging condition. Charging system voltage too high.
47*** Battery voltage sense input below target charging during engine operation. Also, no significant change detected in battery voltage during active test of generator output circuit. – Undercharging condition. Charging system voltage too low.
51** A lean air/fuel mixture has been indicated by an abnormally rich correction factor. – Engine is running lean (too much air, not enough fuel).
52** A rich air/fuel mixture has been indicated by an abnormally rich correction factor. – Engine is running rich (too much fuel, not enough air).
53** PCM Internal fault condition detected. – Indicates a fault within the Powertrain Control Module (PCM) itself. This could be serious and might require PCM replacement or repair.
54** No camshaft signal detected during engine cranking. – Similar to code 11, points to a Camshaft Position Sensor (CMP) issue.
55* Completion of fault code display on Check Engine lamp. – End of diagnostic code display. Normal code to see at the end of a diagnostic sequence.
63* Unsuccessful attempt to write to an EEPROM location by the PCM. – PCM memory issue.
64** Catalyst efficiency below required level. (Same as code 72) / 72** Catalyst efficiency below required level. (Same as code 64) – Catalytic converter efficiency is below the acceptable threshold. Often related to O2 sensor issues or catalytic converter failure.
65* Power steering high pressure seen at high speed. (2.5L only) – Power steering pressure issue, specific to 2.5L engine models.
77* Malfunction detected with power feed to speed control servo solenoids. – Speed control (cruise control) system power supply issue.

Code Severity Indicators:

* = This ERROR CODE will not cause the Check Engine Lamp (MIL) to illuminate.
** = The Check Engine Lamp (MIL) will illuminate during engine operation if this error code is recorded.

Resetting Your Jeep TJ’s Computer (PCM)

After addressing the issue indicated by a trouble code, you’ll want to clear the code and turn off the check engine light. The original article correctly outlines two methods:

Battery Disconnection: Disconnecting the battery will reset the PCM. Crucially, disconnect the negative (ground) terminal first. Leave it disconnected for at least 15-30 minutes to allow the capacitors in the PCM to discharge fully. Reconnect the negative terminal last.
Drive Cycles: If you prefer not to disconnect the battery, the PCM is designed to clear codes automatically after 40 engine start-stop cycles, provided the fault condition is no longer present.

Important Note on Resetting: Resetting the PCM will clear all stored diagnostic trouble codes and also reset the PCM’s learned settings. This might temporarily affect engine performance as the PCM relearns optimal settings.

Conclusion

While the 1997 Jeep TJ’s diagnostic system might seem a bit unique compared to later OBD2 standards, the key cycling method provides a valuable tool for owners to diagnose basic engine issues without needing specialized scan tools. By understanding how to read these two-digit codes and knowing their meanings, you can confidently troubleshoot common problems and keep your 1997 Jeep TJ running smoothly. Remember to consult a repair manual or a professional mechanic for complex issues or if you are unsure about any diagnosis or repair procedure. Happy Jeeping!