Park/Neutral Switch Input Circuit Low
The P0851 code indicates that the Powertrain Control Module (PCM) has detected a low voltage condition in the park/neutral safety switch circuit. This switch tells the PCM whether the transmission is in Park or Neutral, allowing the engine to start. A low input means the circuit voltage is below the expected range, often due to a faulty switch, wiring issues, or a short to ground.
1. Symptoms You Will Notice
- Engine may not start or only starts in Park or Neutral intermittently
- Check Engine Light illuminated
- Transmission may not shift properly
- Cruise control may not engage
2. Most Common Causes
- Faulty park/neutral safety switch
- Short to ground in the switch circuit
- Damaged wiring or connectors
- Corrosion or loose connections
- Failed PCM (rare)
3. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm the code and check for related codes.
Step 2: Visually inspect the park/neutral switch and wiring for damage, corrosion, or loose connections.
Step 3: Test the switch with a multimeter for proper continuity in Park and Neutral positions.
Step 4: Check for voltage at the switch connector; compare to manufacturer specifications.
Step 5: If all else is fine, test the PCM input circuit for shorts or opens.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended. The vehicle may not start or could shift unexpectedly, posing a safety risk.
How much does it cost to fix?▼
Typically $50-$150 for parts and labor, depending on the switch cost and accessibility.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.