P1855

What Does Code P1855 Mean?

The diagnostic trouble code P1855 signifies an “Transmission Transfer Case Contact Plate ‘C’ Open Circuit.” This code is manufacturer-specific, generally indicating an electrical fault within the transfer case’s internal range or mode selection sensing mechanism. The transfer case, a crucial component in 4WD and AWD vehicles, is responsible for distributing power between the front and rear axles and allowing the driver to select various drive modes (e.g., 2WD, 4WD High, 4WD Low, Neutral). To achieve this, it relies on an internal sensor assembly, often referred to as a range sensor, mode switch, or resolver, which typically incorporates multiple electrical contacts or plates that change resistance or continuity based on the transfer case’s selected position.

Contact Plate ‘C’ refers to a specific circuit or segment within this multi-contact sensor assembly. When the Powertrain Control Module (PCM) or Transfer Case Control Module (TCCM) detects an “open circuit” on this specific line, it means there is an interruption in the electrical path. This interruption prevents the module from receiving the expected voltage or ground signal from Contact Plate ‘C’, leading to an out-of-range reading. Essentially, the module cannot accurately determine or confirm the transfer case’s current operational mode or status because the signal from Contact Plate ‘C’ is absent or abnormally high, indicating a break in the circuit rather than a valid signal.

Common Symptoms

• Service 4WD/AWD Light Illumination: The primary indicator, often accompanied by the “Service 4WD” or “4WD System Malfunction” message on the instrument cluster.
• Inability to Shift 4WD Modes: The vehicle may be stuck in 2WD, 4WD High, or 4WD Low, or completely unable to engage any 4WD mode.
• 4WD Indicator Lights Flashing or Incorrect: The 4WD mode indicator lights on the dashboard may flash erratically, remain lit for an incorrect mode, or not illuminate at all.
• Transfer Case “Clunking” or Grinding: While less common for an open circuit, mechanical interference or attempted engagement with an electrical fault can sometimes cause audible noises.
• Reduced Performance or Limp Mode: Some vehicles may enter a limp home mode to protect the drivetrain, potentially limiting speed or engine power.

What Causes the Code P1855?

• Faulty Transfer Case Range Sensor/Contact Plate Assembly: The most common cause, where the internal electrical contacts, traces on the circuit board, or the physical mechanism within the sensor itself wears out, corrodes, or breaks, leading to an open circuit on the ‘C’ contact.
• Damaged Wiring Harness: An open circuit (break) in the wiring leading to or from the transfer case range sensor connector. This can be caused by chafing, rodent damage, impact, or corrosion within the wire strands.
• Corroded Electrical Connectors: Corrosion on the pins or terminals of the transfer case range sensor connector or the TCCM/PCM connector can interrupt the electrical path for Contact Plate ‘C’. Moisture ingress is a frequent culprit.
• Internal Transfer Case Mechanical Issue: While primarily an electrical code, severe internal mechanical wear or damage to the shift fork or actuator mechanism could physically prevent the contact plate from making proper electrical contact, thus appearing as an open circuit.
• Faulty Transfer Case Control Module (TCCM) / Powertrain Control Module (PCM): Less common, but a defective control module could fail to provide the correct reference voltage or improperly interpret the signal from Contact Plate ‘C’, though an “open circuit” usually points to the sensor or wiring.

How to Diagnose and Troubleshoot

Diagnosing P1855 requires a methodical approach, focusing on the electrical integrity of the transfer case range sensor circuit. Always begin with a fully charged battery to avoid spurious voltage-related issues.

1. Verify and Document Codes: Connect an OBD-II scanner to confirm P1855. Check for any other related transfer case, communication, or electrical codes that might provide additional context. Note freeze frame data. Clear the codes and cycle the ignition to see if P1855 immediately reappears.
2. Visual Inspection:
   • Visually inspect the transfer case for any signs of physical damage, fluid leaks, or external wiring damage.
   • Locate the electrical connector for the transfer case range sensor/encoder motor. Inspect the wiring harness from the TCCM/PCM to this sensor for signs of chafing, cuts, pinching, or rodent damage.
   • Carefully disconnect the transfer case range sensor connector and inspect the terminals on both the harness and sensor sides for corrosion, bent pins, pushed-back pins, or loose connections. Clean any corrosion with electrical contact cleaner and a small brush.
3. Electrical Diagnostics with a Digital Multimeter (DMM):
   • Power and Ground Check at Harness Connector: With the ignition ON (engine OFF), use a DMM to back-probe or test the harness side of the transfer case range sensor connector. Consult the vehicle’s specific wiring diagram to identify the reference voltage (usually 5V or 12V) and ground circuits for the sensor. Confirm the presence of proper voltage on the power supply wire and good continuity to chassis ground for the ground wire. An open in either of these fundamental circuits can mimic an open on a specific signal line.
   • Continuity Test of Signal Wire ‘C’: With the ignition OFF and the battery disconnected (or the TCCM/PCM connector disconnected for safety), use the DMM in resistance (Ohms) mode to check the continuity of the signal wire corresponding to Contact Plate ‘C’ from the sensor harness connector back to the TCCM/PCM connector. A reading of “OL” (Over Limit) or very high resistance indicates an open circuit in the wiring itself. Wiggle the harness while testing to identify intermittent breaks.
   • Transfer Case Range Sensor Internal Test: Disconnect the sensor. Refer to the factory service manual for the specific resistance values or voltage outputs expected from the sensor’s various pins in different transfer case positions. If possible, manually rotate the transfer case input shaft (if the sensor is shaft-driven) or manually actuate the sensor’s internal mechanism. Measure the resistance or voltage across the pins corresponding to Contact Plate ‘C’ while attempting to simulate mode changes. An open circuit (OL) or incorrect values on the ‘C’ contact’s circuit confirms an internal sensor fault.
4. TCCM/PCM Verification: If all wiring, connectors, and sensor tests pass, and you’ve confirmed proper power and ground at the sensor, the fault may lie within the TCCM or PCM. This is typically a last resort, and further diagnostics would involve testing the module’s outputs or replacing it, often requiring programming.

Recommended Repairs and Solutions

The recommended repairs for P1855 directly correlate with the diagnostic findings:

• Repair or Replace Damaged Wiring: If the diagnostic steps identify an open circuit within the wiring harness, repair the damaged section using appropriate automotive-grade wire, soldering, and heat-shrink tubing. Ensure connections are secure and weatherproof. If damage is extensive, replace the entire affected harness segment.
• Clean or Replace Corroded Connectors: If corrosion or poor contact is found at the electrical connectors, thoroughly clean the terminals with electrical contact cleaner and a suitable brush. If terminals are severely corroded, bent, or if the connector housing is compromised, replace the connector shell and individual terminals.
• Replace Transfer Case Range Sensor/Contact Plate Assembly: This is a very common solution when the internal diagnostics confirm an open circuit within the sensor itself. Ensure the replacement part is an OEM equivalent or a high-quality aftermarket component that meets manufacturer specifications. Installation typically involves removing the old sensor, installing the new one, and ensuring the transfer case fluid is at the correct level and condition afterward.
• Address Internal Transfer Case Mechanical Issues: If internal mechanical issues are determined to be preventing proper contact plate operation, the transfer case may need to be disassembled for repair or replacement of the affected internal components (e.g., shift fork, detents, gears). This is a more extensive repair.
• Replace Transfer Case Control Module (TCCM) / Powertrain Control Module (PCM): If all other components (wiring, sensor) test good, and a faulty module is suspected, replacement and subsequent programming of the TCCM/PCM may be necessary. Always verify with specific diagnostic routines and, if possible, test with a known good module before replacement.

Mechanic’s Tip: After any repair, always clear the DTCs using an OBD-II scanner. Perform a thorough road test, attempting to engage and cycle through all 4WD modes multiple times, to confirm the repair is successful and the code does not return. Always consult the vehicle’s specific factory service manual for detailed wiring diagrams, pinout configurations, and expected electrical values, as these vary significantly between manufacturers and models. Pay particular attention to the integrity of all ground circuits, as poor grounds can often present as open circuits.