Reverse Input Circuit
Diagnostic trouble code P0812 indicates a malfunction in the reverse input circuit. The powertrain control module (PCM) or transmission control module (TCM) sets this code when it detects an incorrect or missing voltage signal from the reverse gear switch or sensor when the vehicle is shifted into reverse.
While the vehicle can usually still be driven forward, a failure in the reverse input circuit can prevent reverse lights from illuminating, creating a safety hazard. It may also affect backup cameras and parking sensors, so it should be diagnosed and repaired soon.
Common symptoms
- Check Engine Light illuminated
- Reverse lights do not turn on
- Reverse lights stay on continuously
- Backup camera fails to activate when in reverse
- Parking assist systems disabled or malfunctioning
Likely causes
- Faulty reverse light switch or sensor
- Damaged, corroded, or loose wiring in the reverse input circuit
- Blown fuse related to the reverse light circuit
- Misadjusted transmission range sensor or neutral safety switch
- Faulty Powertrain Control Module (PCM) or Transmission Control Module (TCM) (rare)
What to check before replacing parts
Check the reverse light circuit fuse.
Look for: A blown fuse indicates a short circuit or overload in the system that needs to be addressed before replacing the fuse.
Visually inspect the wiring and connectors at the reverse switch.
Look for: Look for frayed wires, corrosion, or loose connections that could interrupt the signal to the PCM/TCM.
Test the reverse light switch for continuity.
Look for: Using a multimeter, verify that the switch closes the circuit when the transmission is shifted into reverse and opens when shifted out. No continuity in reverse indicates a faulty switch.
Check for voltage at the reverse switch connector.
Look for: With the ignition on, verify that the switch is receiving the correct reference voltage (usually 12V). Lack of voltage points to a wiring issue upstream.
Verify the transmission range sensor alignment.
Look for: Ensure the sensor is properly adjusted and accurately reflects the gear selector position on a scan tool data stream.
This page explains the generic P0812 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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