What Does P0851 Mean?
The park/neutral switch input circuit is reading a voltage that is lower than the expected range. This low signal can prevent the TCM/PCM from accurately determining the gear selector position. It typically indicates a short to ground in the wiring, a faulty switch with low resistance, or a connector issue pulling the signal voltage down.
Common Causes
30%
Short to ground in the park/neutral switch input wiring
30%
Faulty park/neutral position switch with low internal resistance
20%
Corroded connector causing high resistance ground path
12%
Damaged internal transmission wiring harness
8%
TCM/PCM input circuit failure
Diagnostic Steps
1
Monitor the park/neutral switch input voltage with the scan tool in live data — compare the reading to manufacturer specifications for each gear position.
2
Disconnect the park/neutral switch connector and measure the input circuit voltage at the TCM/PCM side — if voltage returns to normal, the switch or its wiring is at fault.
3
Check for continuity to ground on the signal wire with the switch disconnected to identify a short to ground.
4
Inspect the wiring harness for chafing against the transmission case or frame components that could cause a ground short.
5
Test the park/neutral switch itself by measuring resistance across its terminals in each position and comparing to specifications.
Estimated Repair Cost
$100 - $400
Parts + labor, varies by vehicle and location
The park/neutral switch input circuit is reading a voltage that is lower than the expected range. This low signal can prevent the TCM/PCM from accurately determining the gear selector position. It typically indicates a short to ground in the wiring, a faulty switch with low resistance, or a connecto...
The most common cause of P0851 (Park / Neutral Switch Input Circuit Low) is: Short to ground in the park/neutral switch input wiring
Typical repair costs for P0851 range from $100 to $400, depending on the vehicle, location, and whether you do it yourself or go to a shop.
The vehicle may exhibit no-start conditions, start in the wrong gear, or display incorrect gear position. The neutral safety function may be compromised, posing a safety risk. Verify the vehicle is safe before driving.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0851 to identify the root cause.
OBDHut Mobile App
Scan codes directly from your car with the OBDHut app.
Coming Soon
Quick Info
Category
Powertrain
System
Transmission / Gear Selection
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
