A park/neutral switch is used to tell the ECU if the transmission has been shifted into park or neutral. If the vehicle has a P0851 DTC, it means that a low voltage signal has been detected on the switch’s input circuit. This code only applies to vehicles that are equipped with an automatic transmission and four wheel drive.
How Serious is the Code?
If the computer cannot tell whether the transmission has been shifted out of gear, it can affect the drivability, as well as the operation of the 4WD system. So a P0851 diagnostic trouble code is actually pretty serious.
Symptoms of a P0851 include:
- 4 Wheel Drive will not engage
- Unusually harsh shifts
- Transmission will not change gears
- Damaged wiring
- Corroded/broken connectors
- Sensor retaining bolts are loose or not installed correctly
- Park/neutral has failed
- Misadjusted transfer case range sensor
- Bad transmission range sensor
How to Diagnose the Code?
When diagnosing a P0851 OBDII trouble code, the first thing the mechanic should do is check the condition of the wiring and connectors. The park/neutral sensor will also need to be tested to make sure that it is properly grounded and pulling the right amount of voltage. If no problems have been found by this point, then the transmission range sensor and transfer case range sensor will need to be checked.
What Repairs Will Fix P0851?
- Repair or replace damaged wiring and connectors
- Replace Park/Neutral switch
- Adjust transfer case range sensor
- Replace transmission range sensor
- Install new sensor retaining bolts
- P0850 – Park/Neutral Switch Input Circuit
P0852 – Park/Neutral Switch Input Circuit High
P0853 – Drive Switch Input Circuit
P0854 – Drive Switch Input Circuit Low