P1851

What Does Code P1851 Mean?

DTC P1851 signifies an “Open Circuit” condition detected within the Transfer Case Contact Plate ‘B’ circuit. This diagnostic trouble code is specific to the vehicle’s four-wheel-drive (4WD) or all-wheel-drive (AWD) system, monitored by either the Powertrain Control Module (PCM) or a dedicated Transfer Case Control Module (TCCM). The transfer case contact plate, often integrated within a range sensor or mode selector switch assembly, is responsible for providing critical positional feedback to the control module regarding the current operating mode of the transfer case (e.g., 2WD, 4HI, 4LO, N). Contact plate ‘B’ refers to a specific contact or switch within this assembly that, when open, should either complete a circuit path or break one to indicate a particular transfer case position. An “open circuit” means the control module is not detecting the expected electrical continuity or signal voltage for contact plate ‘B’ when it should be present, indicating a break in the circuit, a faulty contact, or a wiring issue.

Common Symptoms

• Check Engine Light (CEL) illumination: The primary indicator for most OBD-II related faults.
• 4WD/AWD Warning Light: A dedicated warning light on the instrument cluster may illuminate, indicating a fault with the 4WD/AWD system.
• Inability to Shift Transfer Case: The vehicle may be stuck in a specific drive mode (e.g., 2WD) or unable to engage or disengage 4WD/AWD.
• Erratic Transfer Case Operation: The transfer case may attempt to shift but fail, or exhibit unusual noises during attempts to shift.
• Reduced Power or “Limp Mode”: In some applications, the PCM may limit engine power or vehicle speed to prevent further damage to the drivetrain.
• No transfer case functionality: The 4WD system may be completely inoperable.

What Causes the Code P1851?

• Damaged or Corroded Wiring: The electrical wiring harness leading to the transfer case contact plate sensor can become frayed, chafed, cut, or corroded, leading to an open circuit.
• Faulty Transfer Case Contact Plate/Range Sensor: The internal contacts within the transfer case position sensor or mode selector switch can wear out, become contaminated, or break, resulting in a persistent open circuit for contact ‘B’.
• Corroded or Loose Electrical Connectors: The multi-pin connectors at either end of the wiring harness (at the control module or the transfer case sensor) can suffer from corrosion, bent pins, or inadequate seating, causing intermittent or permanent open circuits.
• Internal Transfer Case Mechanical Issues: While less common for an “open circuit” code, severe internal mechanical damage within the transfer case could prevent the contact plate from being properly actuated, though this usually presents with other mechanical symptoms.
• Control Module Failure (PCM/TCCM): In rare instances, a faulty internal circuit within the Powertrain Control Module or Transfer Case Control Module could incorrectly detect an open circuit, though this is typically a last resort diagnosis.

How to Diagnose and Troubleshoot

Diagnosis of P1851 requires a methodical approach, focusing on electrical circuit integrity and component functionality.

1. Retrieve and Document DTCs: Use an OBD-II scanner to confirm P1851 is present. Note any accompanying codes and retrieve freeze frame data, which can offer clues about vehicle operating conditions when the fault was set.
2. Visual Inspection:
   • Inspect the entire wiring harness leading to the transfer case position sensor from the PCM/TCCM. Look for obvious signs of damage, chafing, cuts, or pinch points, especially where the harness might rub against chassis components or exhaust.
   • Carefully inspect the electrical connectors at both ends of the circuit. Look for corrosion, bent pins, pushed-out pins, or improper seating. Disconnect, clean with electrical contact cleaner, and reconnect.
   • Check for any physical damage to the transfer case itself or the external components of the range sensor.
3. Electrical Circuit Testing with a Digital Multimeter (DMM):
   • Continuity Test: With the ignition off and the control module and transfer case sensor connectors disconnected, identify the specific wire for contact plate ‘B’ using the vehicle’s wiring diagram. Test for continuity through this wire from the control module connector to the transfer case sensor connector. A healthy wire should show near zero ohms. An “open circuit” will show infinite resistance (OL).
   • Resistance Test of the Sensor: If specifications are available, measure the resistance of the contact plate ‘B’ circuit directly at the transfer case position sensor’s connector pins. Compare to manufacturer specifications. An open circuit in the sensor itself will also show infinite resistance. Test the sensor in different transfer case positions if it’s a multi-position switch to verify functionality across all contacts.
   • Voltage Supply and Ground Test: Verify that the transfer case sensor is receiving proper voltage supply (usually 5V or 12V reference) and has a clean ground connection. With connectors reconnected, back-probe the relevant pins at the sensor connector while the ignition is on.
   • Voltage Drop Test: This can help identify resistance in a circuit that isn’t a complete open. Test voltage drop across the positive and ground sides of the circuit under load.
4. Transfer Case Position Test (if accessible): Manually shift the transfer case through its range (e.g., 2HI, 4HI, 4LO) while monitoring the output of the contact plate ‘B’ circuit on a scan tool if it provides live data, or by checking continuity/resistance at the sensor pins for each position (refer to service manual for specific contact plate B logic for each position).
5. Pin-out Verification: Always consult the vehicle’s specific wiring diagrams to correctly identify the wires for contact plate ‘B’ and their corresponding pins at the control module and sensor. Pin-out configurations vary significantly between manufacturers and models.

Recommended Repairs and Solutions

Based on the thorough diagnosis, the following repairs are typically recommended:

• Repair or Replace Damaged Wiring: If the visual inspection or continuity test reveals a damaged wire, repair it using proper automotive wiring repair techniques (e.g., soldering and heat-shrink tubing) or replace the entire section of the wiring harness if damage is extensive.
• Clean and Secure Electrical Connectors: If corrosion or loose connections were found, clean the connector pins with electrical contact cleaner and ensure they are properly seated and locked. Apply dielectric grease to prevent future corrosion.
• Replace the Transfer Case Contact Plate/Range Sensor: If electrical testing confirms an internal fault within the transfer case position sensor or contact plate assembly, the component will need to be replaced. This often requires careful removal of the old sensor and installation of a new one, ensuring proper indexing or alignment according to manufacturer specifications.
• Address Internal Transfer Case Issues: In the rare event that mechanical issues within the transfer case are preventing the contact plate from functioning, internal transfer case repair or replacement may be necessary. This is a complex repair usually performed by a specialized technician.
• Control Module Replacement: Only consider PCM or TCCM replacement as a last resort, after all other circuit and component tests confirm the module is the source of the fault. This typically requires programming the new module to the vehicle.
• Clear DTCs and Test Drive: After any repair, clear the DTCs with a scan tool. Perform a comprehensive test drive, engaging and disengaging the 4WD/AWD system multiple times to ensure the repair is effective and the code does not return.