Park/Neutral Switch Input Circuit Low
What does P0851 mean?
The P0851 code means the vehicle's onboard diagnostic system has detected: Park/Neutral Switch Input Circuit Low. This is a moderate severity code.
Common Symptoms
- Check Engine Light illuminated
- No-start condition in Park
- PCM always reads transmission as in gear
- Starter relay may not engage
Probable Causes (Ranked by Likelihood)
- Short to ground in park/neutral switch wiring Very High
- Faulty park/neutral switch with shorted output High
- Corroded connector at the switch Moderate
- PCM internal pull-down circuit fault Low
Step-by-Step Diagnostic Procedure
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Measure voltage at the park/neutral switch connector with key on
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Check for a short to ground in the signal wire
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Inspect connector for water damage or corrosion
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Verify switch operation by moving shifter through all positions while monitoring live data
Frequently Asked Questions
Why does my car not start in Park with P0851?
A low signal makes the PCM think the transmission is always in gear, so it disables the starter as a safety measure. Try starting in Neutral as a temporary workaround.
Can water cause P0851?
Yes, the park/neutral switch is mounted on the transmission and exposed to road spray. Water intrusion can corrode the connector and cause a short to ground.
Is P0851 an emergency?
While not immediately dangerous, it can leave you unable to start the vehicle. Carry out the repair as soon as possible to avoid being stranded.