Decoding Your Jeep Cherokee’s Brain: Understanding OBDII to PCM Communication

For Jeep Cherokee owners, understanding the onboard diagnostic system, or OBDII, is crucial for vehicle maintenance and troubleshooting. This system acts as your Jeep’s brain, constantly monitoring various sensors and systems, and communicating potential issues through diagnostic trouble codes (DTCs). These codes are your gateway to understanding what might be going wrong under the hood, and often, they point directly to the Powertrain Control Module (PCM), the central computer managing your engine and transmission. This guide will delve into how to retrieve and interpret these codes, specifically focusing on the connection between your Jeep Cherokee’s OBDII system and its PCM.

Understanding the Basics: OBDII and PCM in Your Jeep Cherokee

Modern vehicles like the Jeep Cherokee utilize sophisticated computer systems to manage engine performance, emissions, and various other functions. Two key components in this electronic network are the OBDII system and the PCM.

OBDII (On-Board Diagnostics II): This is a standardized system mandated for all cars sold in the US from 1996 onwards. Its primary purpose is to monitor vehicle emissions systems. However, it has evolved to track a wide array of parameters across the vehicle. When the OBDII system detects an issue, it illuminates the “Check Engine Light” (CEL) or “Malfunction Indicator Lamp” (MIL) on your dashboard and stores a corresponding DTC. This DTC is like a specific error message that mechanics and owners can use to diagnose problems.

PCM (Powertrain Control Module): Often referred to as the engine control unit (ECU) in older contexts, the PCM is the master computer controlling your Jeep Cherokee’s powertrain. It manages critical functions like fuel injection, ignition timing, transmission control, and emissions control based on data received from numerous sensors throughout the vehicle. The PCM is the central processing unit that the OBDII system interacts with to both monitor and report issues.

The OBDII to PCM Connection: The OBDII system is essentially a reporting system that relies on the PCM’s data. Sensors throughout the Jeep send information to the PCM. The PCM analyzes this data, and if it detects a reading outside of pre-programmed parameters, it registers a fault. This fault is then translated into a DTC and stored in the PCM’s memory, accessible through the OBDII port – a standardized connector usually located under the dashboard. Therefore, when you read OBDII codes, you are essentially retrieving information that the PCM has logged about potential issues.

Retrieving Diagnostic Codes on Your Jeep Cherokee: The “Key Trick” Methods

Before the widespread use of dedicated OBDII scanners, manufacturers like Jeep incorporated methods for retrieving basic diagnostic codes directly from the vehicle without special tools. These are often referred to as “key trick” methods. It’s important to note that while these methods can provide valuable initial information, modern OBDII scanners offer much more detailed and accurate diagnostics.

Here are the “key trick” methods for retrieving codes on Jeep Cherokees, covering both older (OBDI) and newer (OBDII) models as described in the original guide.

Method 1: For Older (OBDI) Jeep Cherokee Models

This method is designed for older Jeep Cherokee models equipped with the OBDI system.

1. Start with the Ignition OFF: Ensure your Jeep Cherokee’s ignition is completely off.
2. Key Cycling Sequence (Within 5 Seconds): Perform the following sequence rapidly within five seconds:
   • Turn the key to the “ON” position (do not start the engine).
   • Turn the key back to the “OFF” position.
   • Turn the key to the “ON” position again.
   • Turn the key back to the “OFF” position.
   • Turn the key to the “ON” position one last time.
     (Note: “ON” is the position right before “Start,” and “OFF” is not “Lock” but the standard off position.)
3. Observe the “Check Engine” Light: The “Check Engine” light on your dashboard will begin to flash.
4. Count the Flashes: Carefully count the flashes. OBDI codes are typically two-digit codes.
   • A code like “23” would be represented as: FLASH FLASH (short pause) FLASH FLASH FLASH (long pause).
   • Pay close attention to the pauses to differentiate between digits and separate codes.
   • The code “55” (FLASH FLASH FLASH FLASH FLASH – long pause – FLASH FLASH FLASH FLASH FLASH) indicates the “end of codes” and is a normal signal.

Important Notes for OBDI Method:

• Parking Brake: In some older models, you may need to engage the parking brake for the codes to display.
• Blink Spacing: The timing of the flashes might not be perfectly uniform on older systems. If you see a sequence like “23 23,” it likely indicates a single code “55” (end of codes) due to slight variations in timing. Codes are not typically repeated in this method.

Method 2: For Newer (OBDII) Jeep Cherokee Models

This method is designed for newer Jeep Cherokee models equipped with the OBDII system and often featuring a digital odometer.

1. Insert Key and Locate Odometer Reset Button: Insert the key into the ignition switch. Locate the odometer reset button, usually a small button protruding from the instrument cluster.
2. Press and Hold Odometer Reset Button: Push and hold the odometer reset button down.
3. Turn Key to “RUN” Position: While holding the odometer reset button, turn the ignition key to the “RUN” position (the position before starting the engine).
4. Release Odometer Reset Button: Release the odometer reset button.
5. Observe the Odometer Display: The odometer display will go through a sequence of numbers, typically cycling from “111111” to “999999.” It may then display the vehicle’s serial number. After this sequence, it should display any stored error codes.
   • OBDII codes displayed digitally often start with a letter, such as “P” (for Powertrain codes).

Alternative Method for Newer Models (OBDI Method Attempt):

If the OBDII method above does not work on your newer Jeep Cherokee, you can try the OBDI key cycling method described earlier. Some newer models may still respond to this older method, although it’s less common.

Using an OBDII Scanner: The Modern Approach

While the “key trick” methods can be helpful in a pinch, using a dedicated OBDII scanner is the most reliable and user-friendly way to retrieve and interpret diagnostic codes from your Jeep Cherokee.

Benefits of using an OBDII Scanner:

• Accuracy and Detail: Scanners provide precise code readings and often offer more detailed descriptions of the fault.
• Code Clearing: Scanners allow you to clear codes after addressing the underlying issue, turning off the “Check Engine Light.”
• Live Data Streaming: Many scanners can display real-time data from vehicle sensors, providing valuable insights for diagnosis.
• Advanced Functions: Some scanners offer advanced features like freeze frame data (a snapshot of sensor readings when the code was set) and access to manufacturer-specific codes beyond the standard OBDII codes.

OBDII scanners are readily available at auto parts stores and online retailers, ranging from basic, affordable models to professional-grade tools. Investing in a scanner can be a worthwhile investment for any Jeep Cherokee owner who prefers to handle their own vehicle maintenance.

Understanding Jeep Cherokee Diagnostic Codes: OBDI and OBDII Code Lists

Once you have retrieved the diagnostic codes from your Jeep Cherokee using either the “key trick” method or an OBDII scanner, the next step is to understand what these codes mean. The following lists provide a comprehensive guide to both older OBDI codes and newer OBDII codes, as compiled from various sources. It’s important to remember the disclaimers from the original source: these lists are aids for troubleshooting and not definitive repair manuals. Always consult a qualified mechanic for accurate diagnosis and repairs.

Older Model Computer Codes (OBDI) for Jeep Cherokee

(Note: Codes may vary for vehicles circa 1996 onwards. Always verify code definitions specific to your Jeep Cherokee’s year and model.)

• 11: No ignition reference signal detected during cranking OR timing belt skipped one or more teeth; OR loss of either camshaft or crankshaft position sensor.
• 12: Battery or computer recently disconnected. (This code is common after battery replacement or service and may not indicate a fault if the vehicle is operating normally.)
• 13: MAP sensor or vacuum line may not be working.
• 14: MAP sensor voltage below .16V or over 4.96V.
• 15: No speed/distance sensor signal.
• 16: Loss of battery voltage detected with engine running.
• 17: Engine stays cool too long (bad thermostat or coolant sensor?) OR 17 (1985 turbo only): knock sensor circuit.
• 21: Oxygen sensor signal doesn’t change (stays at 4.3-4.5V). Probably bad oxygen sensor.
• 22: Coolant sensor signal out of range – May have been disconnected to set timing.
• 23: Incoming air temperature sensor may be bad.
• 24: Throttle position sensor over 4.96V.
• 25: Automatic Idle Speed (AIS) motor driver circuit shorted or target idle not reached.
• 26: Peak injector circuit voltage has not been reached (need to check computer signals, voltage reg, injectors).
• 27: Injector circuit isn’t switching when it’s told to (TBI) OR (MPI) injector circuit #1 not switching right OR (turbo) injector circuit #2 not switching right OR (all 1990-) injector output driver not responding – check computer, connections.
• 31: Bad evaporator purge solenoid circuit or driver.
• 32: (1984 only) power loss/limited lamp or circuit OR 32 EGR gases not working (1988): check vacuum, valve OR 32 (1990-92): computer didn’t see change in air/’fuel ratio when EGR activated – check valve, vacuum lines, and EGR electrical.
• 33: Air conditioning clutch relay circuit open or shorted (may be in the wide-open-throttle cutoff circuit or normal on early 80’s models if you don’t have air conditioning).
• 34: (1984-86) EGR solenoid circuit shorted or open OR 34 (1987-1991): speed control shorted or open.
• 35: Cooling fan relay circuit open or shorted OR 35 idle switch motor fault – check connections.
• 36 (turbo only): Wastegate control circuit open or shorted.
• 37: Shift indicator light failure, 5-speed OR part throttle lock/unlock solenoid driver circuit (87-89) OR solenoid coil circuit (85-87 Turbo only) OR Trans temperature sensor voltage low (1995 and on).
• 41: Alternator field control circuit open or shorted.
• 42: Automatic shutdown relay circuit open or shorted OR 42 Fuel pump relay control circuit OR 42 Fuel level unit – no change over miles OR 42 Z1 voltage missing when autoshutdown circuit energized (The Z1 voltage is the voltage of the circuits fed by the autoshutdown relay. This typically includes fuel pump and switched-battery feed to the ignition coil).
• 43: Peak primary coil current not achieved with max dwell time OR 43 Cylinder misfire OR 43 Problem in power module to logic module interface.
• 44: No FJ2 voltage present at logic board OR 44 Logic module self-diagnostics indicate problem OR 44 Battery temperature out of range.
• 45: Turbo boost limit exceeded (engine was shut down by logic module).
• 46: Battery voltage too high during charging or charging system voltage too low.
• 47: Battery voltage too low and alternator output too low.
• 51: Oxygen sensor stuck at lean position (this may be tripped by a bad MAP sensor system causing a rich condition. If you get hot rough idle and stalling, especially on deceleration, accompanied by flooded engine and difficulty restarting, that can be a bad MAP sensor causing the O2 sensor to try to compensate. If you get poor cold driveability, stumbling and bucking, and acceptable warm driving with poor gas mileage (a drop of 10 mpg or more), that is usually the O2 sensor. OR 51 Internal logic module fault (’84 turbo only).
• 52: Oxygen sensor stuck at rich position OR 52 Internal logic module fault (’84 turbo only).
• 53: Logic module internal problem.
• 54: No sync pickup signal during engine rotation (turbo only) OR 54 Internal logic module fault (’84 turbo only) – or camshaft sensor/distributor timing.
• 55: End of codes (Normal – indicates the system has finished displaying codes).
• 61: “Baro” sensor open or shorted.
• 62: EMR mileage cannot be stored in EEPROM OR 62 PCM failure SRI mile not stored.
• 63: Controller cannot write to EEPROM.
• 64: Catalytic converter efficiency failure.
• 65: Power steering switch failure.
• 66: No CCD Message From TCM and/or No CCD Message From BCM.

Newer Model Computer Codes (OBDII) for Jeep Cherokee

(Note: OBDII codes are standardized across manufacturers, but always verify the specific code definition for your Jeep Cherokee’s year and model for the most accurate diagnosis.)

• P0030, P0036, P0130, P0136, P0142, P0147, P0155, P0161: Sensor heater relay problem / O2 Sensor Circuit Malfunction (Bank 1 Sensor 1, Bank 1 Sensor 2, Bank 1 Sensor 3, Bank 2 Sensor 1, Bank 2 Sensor 2, Bank 2 Sensor 3).
• P0106, P0107, P0108: MAP sensor voltage out of range detected at startup / MAP sensor voltage too low / MAP sensor voltage too high.
• P0112, P0113: Intake air temperature sensor voltage low / Intake air (charge) temperature sensor voltage high.
• P0116: Coolant temperature sensor reading doesn’t make sense.
• P0117, P0118: Engine coolant temperature sensor voltage low / Engine coolant temperature sensor voltage high.
• P0121: Throttle position sensor and MAP sensor disagree with each other.
• P0122, P0123: Throttle position sensor voltage low / Throttle position sensor voltage high.
• P0125: Taking too long to reach proper operating temperature and switch to energy-efficient mode.
• P0131, P0132, P0133, P0134, P0137, P0138, P0139, P0140, P0143, P0144, P0145, P0146: O2 Sensor Circuit Low/High Voltage, Slow Response, No Activity Detected (Bank 1 & 2, Sensor 1, 2, & 3).
• P0151, P0152, P0153, P0154, P0157, P0158, P0159, P0160: Oxygen sensor voltage problem – short circuit to ground/active 12V? / Oxygen sensor response too slow / Oxygen sensor does not show either a rich or lean condition.
• P0171, P0172, P0174, P0175: System too lean (Bank 1 & 2) / System too rich (Bank 1 & 2).
• P0176, P0178, P0179: Flex fuel sensor can’t be seen / Flex fuel sensor problem.
• P0182, P0183: Compressed natural gas temperature sensor problem.
• P0201, P0202, P0203, P0204, P0205, P0206, P0207, P0208: Injector #1-8 control circuit problem (open or shorted).
• P0300: Misfire detected in multiple cylinders.
• P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308: Misfire detected in cylinder #1-8.
• P0320: Crankshaft position sensor reference signal cannot be found while the engine is cranking.
• P0325, P0330: Knock sensor (#1 & #2) signal is wrong.
• P0340: No camshaft signal being received by the computer.
• P0350: A coil is drawing too much current.
• P0351, P0352, P0353, P0354, P0355, P0356, P0357, P0358: Coil #1-8 is not reaching peak current at the right time.
• P0401: A required change in air/fuel mixture was not detected during diagnostic test (EGR).
• P0403: An problem was detected in the EGR solenoid control circuit.
• P0404, P0405, P0406: The EGR sensor’s reported position makes no sense / EGR position sensor voltage wrong.
• P0412: The secondary air solenoid control circuit seems bad (aspirator).
• P0420, P0432: The catalyst seems inefficient (#1 & #2).
• P0441, P0442, P0443, P0455, P0456: Evaporative purge flow system not working properly / A leak has been detected in the evaporative system! (Small/Large leak).
• P0460, P0461, P0462, P0463: The fuel level sender is not reporting any change over a long distance/time / Fuel level sender voltage wrong.
• P0500: Haven’t heard from the speed sensor lately.
• P0505: The idle speed air control motor doesn’t seem to be working correctly.
• P0522, P0523: Oil pressure sensor problems.
• P0551: The power steering switch may not be working. (Neons: high pressure is showing up at high speed).
• P0600, P0601, P0604, P0605: Oh-oh! The coprocessors aren’t talking to each other within the computer! / Internal computer error! (RAM/ROM check).
• P0615: Starter relay circuit problem.
• P0622: Generator field control problem.
• P0645: A/C clutch relay circuit problem.
• P0700: The automatic transmission computer or Aisin computer has a problem – ask it what’s going on. I don’t know.
• P0703: Brake switch circuit information seems wrong.
• P0711, P0712, P0713: Based on the transmission temperature and its operations, it looks like the transmission temperature sensor’s gone bad / Transmission fluid temperature sensor voltage wrong.
• P0720: The Output Shaft Speed Sensor doesn’t match the reported vehicle speed.
• P0740: The engine’s running faster than it should for these speeds, so I think the torque convertor clutch lock-up system is bad.
• P0743: Torque converter clutch (part throttle unlock) solenoid circuit problem – shift solenoid C electrical fault.
• P0748: Governor Pressure Solenoid circuit problem (Transmission relay circuit problem in Jeep RE transmissions).
• P0751: Overdrive override switch has been pressed for over five minutes. Just thought you should know.
• P0753: Overdrive solenoid control circuit problem (transmission relay circuit in Jeep RE transmissions.).
• P0756: Shift solenoid B (2-3) fault.
• P0783: The overdrive solenoid can’t go from 3rd gear to the overdrive gear.
• P0801: Transmission reverse gear lockout solenoid circuit problem!
• P0833: Problem with the clutch-released switch circuit?
• P1192, P1193: Inlet air temperature sensor voltage is wrong.
• P1194: Oxygen sensor heater performance is faulty.
• P1195, P1196, P1197: Oxygen sensor is slow / The oxygen sensor switched too slowly (bank 2/1).
• P1198, P1199: Radiator coolant temperature sensor voltage is wrong.
• P1281: The engine is staying cold too long – check your thermostat.
• P1282: The fuel pump relay circuit seems to be having a problem.
• P1288: The intake manifold short runner tuning valve circuit seems to be having a problem.
• P1289: There’s a problem in the manifold tuning valve solenoid control circuit.
• P1290: Compressed natural gas system pressure is too high.
• P1291: The heated air intake sensor does not seem to be working.
• P1292, P1293: Natural gas pressure sensor issue.
• P1294: Can’t get to target engine speed, check for vacuum leaks and idle speed motor issues.
• P1295: The throttle position sensor doesn’t seem to be getting enough electricity.
• P1296: The MAP sensor doesn’t seem to be getting enough electricity.
• P1297: The MAP sensor doesn’t change its reading when the engine is running!
• P1298: During wide-open throttle, the engine runs lean.
• P1299: MAP Sensor and Throttle Position Sensor signals don’t match, check for a vacuum leak.
• P1388, P1389: Auto shutdown relay circuit problems? / No Z1 or Z2 voltage seen by the computer when the auto shutdown relay is used.
• P1390: Cam and crank signals don’t match – did the timing belt skip a tooth?
• P1391: Sometimes, I can’t see the crank or cam sensor signal.
• P1398: I can see the Crank Sensor’s signal when I prepare for Misfire Diagnostics. Try replacing it.
• P1399: Problem in the Wait to Start Lamp circuit – (diesels only?).
• P1403: EGR position sensor not getting (enough) voltage.
• P1476, P1477: Too little/much secondary air injection during aspirator test.
• P1478, P1479: Battery temperature sensor voltage wrong / Transmission fan relay circuit problems?
• P1480: PCV solenoid circuit problems?
• P1481: Transmission RPM pulse generator signal for misfire detection seems wrong.
• P1482, P1483, P1484: Catalyst temperature sensor circuit shorted low/high / The catalyst seems to be overheating!
• P1485: Air injection solenoid circuit problems.
• P1486: Pinched or blocked hose in the evaporative hose system.
• P1487: Control circuit of the #2 high-speed radiator fan control relay is having problems.
• P1488: Auxiliary 5-volt sensor feed is too low.
• P1489, P1490, P1491: High/Low speed radiator fan control circuit problem / Radiator fan control circuit problem.
• P1492, P1493: Ambient or battery temperature sensor voltage wrong.
• P1494, P1495: Leak Detection Pump (LDP) pressure switch problem – electrical or the pump itself / Leak Detection Pump (LDP) pressure switch problem – the solenoid circuit.
• P1496: Sensor feed is below an acceptable limit. (under 4v for 4 seconds – should be 5v).
• P1498: High speed radiator fan control circuit problem. (#3 control relay).
• P1594: Voltage too high in charging system.
• P1595, P1596, P1597: Speed control vacuum or vent solenoid control circuits shorted or lost / Speed control switch always high/low.
• P1598, P1599: A/C pressure sensor voltage high/low.
• P1681: No messages received from the cluster control module (dashboard computer).
• P1682: Charging system doesn’t seem to be working well. Check alternator, etc.
• P1683: Speed control servo power control circuit problem.
• P1684: The battery has been disconnected within the last 50 starts. (This code is common after battery service and may not indicate a fault if the vehicle is operating normally.)
• P1685, P1686: Invalid key received from/No messages received from the Smart Key Immobilizer Module.
• P1687: No messages received from the Mechanical Instrument Cluster module.
• P1693: The companion engine control module has shown a fault.
• P1694, P1695, P1696, P1697, P1698: No messages received from the powertrain control module-Aisin transmission / No messages received from the body control module / Unsuccessful attempt to write to an EEPROM location! / Unsuccessful attempt to update Service Reminder Indicator (SRI or EMR) mileage! / No messages received from the electronic transmission control module or the Aisin transmission controller.
• P1719: Transmission 2-3 gear lockout solenoid control circuit problem.
• P1740: Either the tcc solenoid or overdrive solenoid systems doesn’t seem to be making much sense.
• P1756, P1757: Transmission control pressure not equal to target. (Mid/Zero pressure problem).
• P1762, P1763, P1764: The Governor Pressure Sensor input was too high or too low for 3 consecutive park/neutral calibrations / The Governor Pressure Sensor input is too high/low.
• P1765: Open or short in the Transmission Relay control circuit.
• P1899: The Park/Neutral switch seems to be stuck!

Understanding OBDII Codes in Relation to PCM Issues

Many of the OBDII codes listed above, especially those related to sensors, actuators, and circuits, can directly indicate potential issues with the PCM or its communication with various components.

For example:

• Sensor circuit codes (e.g., MAP sensor, TPS, O2 sensor codes): While a code might indicate a faulty sensor, it could also point to a problem with the PCM’s ability to read signals from that sensor, potentially due to wiring issues or internal PCM faults.
• Actuator circuit codes (e.g., injector codes, solenoid codes): Similarly, these codes could indicate a problem with the actuator itself, or a failure in the PCM’s driver circuit that controls the actuator.
• Communication codes (e.g., codes indicating “no message from…”): These codes often directly point to PCM communication problems, either with other modules in the vehicle’s network or internal PCM communication failures.
• Internal PCM error codes (e.g., P0600, P0601, P0604, P0605, OBDI codes 53, 62, 63): These codes are the most direct indicators of potential PCM failure, suggesting internal processing, memory, or communication issues within the PCM itself.

When diagnosing OBDII codes on your Jeep Cherokee, it’s crucial to consider the possibility of PCM involvement, especially if you encounter:

• Multiple seemingly unrelated codes: If several codes appear simultaneously, affecting different systems, it could indicate a central PCM issue rather than multiple independent component failures.
• Codes that reappear after component replacement: If you replace a sensor or actuator based on a code, but the code returns shortly after, the underlying problem might be with the PCM’s control or interpretation of signals.
• Intermittent or erratic symptoms: PCM issues can sometimes manifest as intermittent problems that are difficult to pinpoint.

Disclaimer: The information provided in this guide is intended for informational purposes and as an aid in troubleshooting. It is not a substitute for professional automotive diagnostic services and repair manuals. Diagnosing and repairing modern vehicle systems can be complex and requires specialized knowledge and tools. Always consult a qualified and experienced mechanic for accurate diagnosis and repairs, particularly when dealing with potential PCM or electrical system issues. Improper diagnosis or repairs can lead to further vehicle damage or safety hazards.

This guide is compiled from various sources and is intended to be as comprehensive as possible. However, accuracy cannot be guaranteed, and vehicle-specific information may vary. Always refer to your Jeep Cherokee’s service manual and consult with a professional for definitive diagnostic and repair procedures.