Park/Neutral Switch Input Circuit Low
The P0851 code indicates that the powertrain control module (PCM) has detected a low voltage signal from the park/neutral position (PNP) switch circuit. This switch tells the PCM whether the transmission is in Park or Neutral, which affects idle speed, cruise control, and starting. A low input typically means the circuit voltage is below the expected threshold, often due to a short to ground, faulty switch, or wiring issues.
01. Most Common Causes
- Faulty park/neutral position switch
- Short to ground in the PNP switch circuit
- Corroded or damaged wiring/connectors
- Failed PCM (rare)
02. Symptoms You Will Notice
- Engine may not start or starts only in Park or Neutral
- Check Engine Light illuminated
- Erratic idle or stalling when shifting
- Cruise control inoperative
03. Step-by-Step Diagnosis
Step 1: Scan for codes and verify P0851 is present.
Step 2: Visually inspect the PNP switch and wiring for damage or corrosion.
Step 3: Test the PNP switch signal voltage with a multimeter; should be near battery voltage in Park/Neutral.
Step 4: Check for continuity to ground; if shorted, repair wiring or replace switch.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It may be unsafe if the engine stalls or fails to start. Driving with a faulty PNP switch can also affect transmission operation. It's best to diagnose and repair promptly.
How much does it cost to fix?▼
Typically $50-$150 for a new switch plus labor. Wiring repairs may cost less, but PCM replacement is more expensive.