Reverse Input Circuit Malfunction
The P0812 code indicates a problem in the reverse input circuit, which is part of the transmission range sensor or switch. This circuit sends a signal to the powertrain control module (PCM) when the transmission is in reverse. A fault can be due to a defective reverse switch, wiring issues, or a faulty PCM. The PCM detects an incorrect voltage or signal from the circuit, triggering the code.
1. Symptoms You Will Notice
- Reverse lights not working
- Transmission may not shift into reverse properly
- Check Engine Light illuminated
- Possible transmission performance issues
2. Most Common Causes
- Faulty or misadjusted transmission range (TR) switch
- Open, shorted, or high-resistance wiring in the reverse input circuit
- Corroded, loose, or damaged reverse switch connector
- Internal PCM/TCM failure (less common)
- Mechanical issues in the transmission gear selector mechanism
- Blown fuse related to the reverse circuit
3. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm the code and check for related codes.
Step 2: Inspect the reverse light switch and circuit for damage, corrosion, or loose connections.
Step 3: Test the reverse switch with a multimeter for continuity when in reverse.
Step 4: Check wiring and connectors from the switch to the PCM for shorts or opens.
Step 5: If all else is fine, test the PCM input/output for the reverse circuit.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It may be safe for short distances, but driving without reverse lights is illegal and could cause an accident. Also, transmission shifting may be affected, so it's best to diagnose and repair promptly.
How much does it cost to fix?▼
Typically $50-$150 for parts and labor, depending on the cause. A switch replacement is usually under $100, while wiring repairs may cost more.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.