P0708

What Does Code P0708 Mean?

DTC P0708 signifies a Transmission Range Sensor Circuit High Input. This diagnostic trouble code indicates that the Powertrain Control Module (PCM), or in some architectures, the Transmission Control Module (TCM), has detected an abnormally high voltage signal from the Transmission Range Sensor (TRS), also commonly referred to as the Neutral Safety Switch or PRNDL switch. The TRS is a critical input device that informs the PCM/TCM of the driver’s selected gear position (Park, Reverse, Neutral, Drive, Low gears). The PCM/TCM expects to see specific voltage ranges or resistance values corresponding to each gear position. A “high input” implies that the received voltage is consistently above the maximum expected threshold for any valid gear state, or it remains high when a change to a lower voltage or open circuit state is expected, suggesting an electrical anomaly such as an open circuit in the sensor’s internal resistance path, a short to a voltage source in the circuit, or a sensor malfunction preventing it from outputting the correct low-voltage signals.

This anomaly prevents the control module from accurately determining the transmission’s current gear, impacting critical functions such as engine starting (via the neutral safety interlock), transmission shift logic, torque converter clutch operation, reverse lamp activation, and dashboard gear display. The PCM/TCM typically sets the P0708 code when it perceives a voltage signal that is out of its calibrated high-end operating range for a specified period, flagging a circuit integrity issue rather than a specific gear position error.

Common Symptoms

• Engine No-Start Condition: The engine may not crank or start, as the PCM may incorrectly believe the transmission is not in Park or Neutral, activating the neutral safety switch function.
• Transmission Stays in Limp Mode: The transmission may enter a failsafe or “limp home” mode, often remaining in a single gear (e.g., 2nd or 3rd) to prevent damage.
• Incorrect Gear Display: The instrument cluster’s PRNDL indicator may display an incorrect gear (e.g., ‘N’ when in ‘D’), illuminate no gear, or flash erratically.
• Reverse Lights Inoperative: The reverse lights may not illuminate when the shifter is placed in ‘R’.
• Harsh or Delayed Shifting: Transmission shifts may be erratic, harsh, or significantly delayed due to incorrect gear position data.
• Cruise Control Inoperative: The cruise control system may become disabled as a safety measure if the PCM cannot confirm the transmission’s state.
• Erratic Speedometer/Tachometer Readings: In some rare cases, related circuit issues can affect other sensor readings, though this is less common for P0708 directly.

What Causes the Code P0708?

• Faulty Transmission Range Sensor (TRS): Internal failure of the sensor, such as an open circuit in its internal resistive track, damaged internal contacts, or a short that causes a perpetually high output voltage.
• Wiring Harness Issues:
  • Open Circuit: A break or discontinuity in the signal wire connecting the TRS to the PCM/TCM.
  • Short to Voltage: The TRS signal wire making accidental contact with a positive voltage source (e.g., battery positive, ignition voltage), causing a consistently high input.
  • Corrosion or Damage: Significant corrosion, fraying, or physical damage to the wiring harness or connector pins leading to the TRS.
  • Poor Connection: Loose or corroded terminal pins within the TRS connector or the PCM/TCM connector.
• Powertrain Control Module (PCM) / Transmission Control Module (TCM) Failure: A rare but possible internal defect within the control module’s input circuit that misinterprets the TRS signal or fails to properly pull down the signal voltage.
• Improper TRS Adjustment (if applicable): On vehicles where the TRS is mechanically adjustable, incorrect alignment can sometimes lead to signals that are out of expected range, though this often manifests as incorrect gear indication or different DTCs rather than a strict “high input” unless severely misaligned causing an open circuit.

How to Diagnose and Troubleshoot

Diagnosing P0708 requires a systematic approach using specialized tools and a thorough understanding of automotive electrical systems.

1. Visual Inspection:
   • Locate the Transmission Range Sensor. It is typically mounted on the side of the transmission housing, often where the shift linkage connects.
   • Carefully inspect the TRS and its electrical connector for signs of physical damage, corrosion, bent pins, or loose connections.
   • Trace the wiring harness from the TRS towards the PCM/TCM, looking for chafing, cuts, signs of rodent damage, or any areas where the wiring could be shorting to chassis ground or another power source.
2. OBD-II Scanner Live Data Analysis:
   • Connect an advanced OBD-II scanner capable of displaying live data.
   • Monitor the “Transmission Range Sensor” or “PRNDL Switch” parameter in the live data stream.
   • With the ignition ON (engine OFF), cycle the transmission shifter through all gear positions (P, R, N, D, L/2/1) while observing the scanner’s reported gear.
   • If the scanner consistently reports an incorrect or fixed high voltage reading, or fails to change its reported gear position accurately, it confirms a problem in the TRS circuit. Pay close attention to any indication of an “implausible” gear or a value that does not correspond to any valid gear.
3. Digital Multimeter (DMM) Testing:
   • Power and Ground at TRS Connector:
     • With the TRS connector disconnected, turn the ignition ON.
     • Using a DMM, check for the presence of battery voltage (B+) or a 5-volt reference signal (depending on the TRS design) at the appropriate power supply pin of the connector.
     • Verify a good ground connection at the ground pin of the connector using the DMM’s continuity or resistance function to chassis ground.
   • TRS Signal Wire Test (Disconnected Sensor):
     • With the TRS connector still disconnected from the sensor and the ignition ON, backprobe the signal wire(s) leading to the PCM/TCM.
     • If you read a high voltage (e.g., near battery voltage) on the signal wire when the sensor is disconnected, it strongly suggests a short to voltage within the wiring harness between the TRS connector and the PCM/TCM. This is a common cause for P0708.
   • TRS Sensor Resistance/Continuity Test (Sensor Disconnected):
     • Disconnect the TRS sensor from the wiring harness.
     • Consult the vehicle’s service manual for the specific pinout and expected resistance values or continuity patterns for the TRS.
     • Using the DMM in resistance (ohms) or continuity mode, test between the specified pins of the TRS as you manually rotate the sensor shaft (if accessible) or have an assistant shift the transmission through each gear position.
     • Look for an open circuit (OL or excessively high resistance) where a specific resistance value or continuity is expected for a given gear, especially in positions that should cause a lower voltage output. A faulty sensor will often show inconsistent or incorrect resistance values.
   • Wiring Harness Continuity & Short Tests (PCM/TCM Disconnected):
     • With the TRS connector disconnected AND the PCM/TCM connector disconnected (CAUTION: disconnect battery before unplugging PCM/TCM), test the continuity of each signal wire from the TRS connector to its corresponding pin at the PCM/TCM connector. Look for very low resistance (ideally < 1 ohm).
     • Test each signal wire for a short to ground (continuity between the signal wire and chassis ground) and a short to voltage (continuity between the signal wire and any power wire in the harness). Any continuity to ground or power where none should exist indicates a short.
4. PCM/TCM Input Circuit Test: This should only be considered after thoroughly verifying the sensor and wiring. This typically involves specialized factory diagnostic tools and may require a qualified technician.

Recommended Repairs and Solutions

Based on the diagnostic findings, the following repairs are typically recommended:

1. Replace the Transmission Range Sensor (TRS): If DMM testing confirms an internal fault with the TRS (incorrect resistance values, open circuits), replacement is the most common solution. Ensure the new sensor is properly aligned or adjusted according to manufacturer specifications upon installation. Some vehicles require careful indexing of the sensor to the transmission shift shaft.
2. Repair or Replace Wiring Harness: If diagnostic steps identify an open circuit, a short to voltage, or extensive damage/corrosion in the wiring harness, the affected section must be repaired or replaced.
   • For minor damage, use appropriate automotive-grade wire repair kits, solder connections, and shrink tubing.
   • For extensive damage or difficult-to-isolate shorts, replacing the entire sub-harness may be necessary.
3. Clean or Replace Connectors: If corrosion or poor terminal tension is found in the TRS or PCM/TCM connectors, clean the terminals thoroughly using electrical contact cleaner and a terminal pick. If damage is severe, replace the connector shell and terminals. Apply dielectric grease to prevent future corrosion.
4. PCM/TCM Replacement/Reprogramming: If all other components (TRS, wiring, connectors) test good, and no external fault can be found, a faulty PCM/TCM input circuit could be the culprit. This is a rare occurrence for P0708. If a replacement module is installed, it typically requires programming or flashing to the vehicle’s specific VIN and options.

After any repair, clear the DTCs using an OBD-II scanner. Perform a test drive, ensuring the transmission shifts correctly and the gear indicator functions as expected. Recheck for any pending or stored codes to confirm the repair’s effectiveness.