Reverse Output Circuit
Diagnostic trouble code P0813 indicates a malfunction in the reverse output circuit, which is responsible for signaling when the vehicle is in reverse gear. The Powertrain Control Module (PCM) or Transmission Control Module (TCM) sets this code when it detects an improper voltage or resistance level in the reverse output circuit.
While this code typically does not affect the drivability of the vehicle, it can disable reverse lights and backup cameras, creating a safety hazard when backing up. It should be diagnosed and repaired soon to ensure safe operation.
Common symptoms
- Illuminated Check Engine Light
- Reverse lights do not illuminate
- Reverse lights stay on continuously
- Backup camera fails to activate when in reverse
- Parking assist sensors fail to activate in reverse
Likely causes
- Faulty reverse light switch or sensor
- Damaged, corroded, or loose wiring in the reverse output circuit
- Blown fuse related to the reverse light circuit
- Faulty transmission range sensor
- Internal failure in the Powertrain Control Module (PCM) or Transmission Control Module (TCM)
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.
Inspect the wiring and connectors to the reverse light switch.
Look for: Look for frayed wires, corrosion, or loose connections that could cause an open or short circuit.
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 it when shifted out. Lack of continuity indicates a faulty switch.
Check the transmission range sensor alignment and operation.
Look for: Ensure the sensor correctly identifies when the transmission is in reverse. Incorrect readings suggest a misaligned or failed sensor.
Test the circuit for voltage from the PCM/TCM to the reverse switch.
Look for: Verify that the correct reference voltage is reaching the switch. No voltage indicates a wiring issue between the module and the switch, or a faulty module.
This page explains the generic P0813 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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