Faulty or misadjusted park/neutral position switch (or transmission range sensor); Selector cable or linkage out of adjustment so the switch does not match the lever position; Damaged, chafed or shorted wiring in the switch signal circuit; Corroded, loose or water-damaged connector at the switch; Open or poor ground, or a supply fault affecting the switch circuit; Fault within the powertrain control module or transmission control module input.

P0851

Park/Neutral Switch Input Circuit Low.

Generic
Powertrain
Generic code
Medium severity

How this code is built

P: SystemPowertrain
0: Defined bySAE, the same on every car
8: AreaTransmission
51: FaultThe specific fault in that area

What it means

P0851 means the powertrain control module has detected a low voltage signal in the park/neutral switch input circuit. The switch tells the control unit whether the gearbox is in Park or Neutral, and the reported state does not match what the module expects, or the circuit voltage is lower than it should be. The exact switch type and the way it is monitored vary by vehicle and gearbox.

Symptoms

Engine cranks but will not start, or will only start in one selector position
Starter does not engage in Park or Neutral
Reversing lights not working or staying on
Rough or unstable idle when selecting Drive or Reverse
Harsh or delayed gear engagement
Malfunction indicator lamp on, possibly with a gear selector or transmission warning

Possible causes

Faulty or misadjusted park/neutral position switch (or transmission range sensor)likely
Selector cable or linkage out of adjustment so the switch does not match the lever positionlikely
Damaged, chafed or shorted wiring in the switch signal circuitpossible
Corroded, loose or water-damaged connector at the switchpossible
Open or poor ground, or a supply fault affecting the switch circuitpossible
Fault within the powertrain control module or transmission control module inputrare

How to diagnose

Read the stored codes and freeze frame data, and note any related gearbox or starter codes
Check the live data for the park/neutral switch status and compare it with the actual selector position in Park, Reverse, Neutral and Drive
Inspect the switch, wiring and connector for damage, corrosion, moisture or contamination from fluid
Check the selector cable and linkage for correct adjustment and wear, following the manufacturer's procedure for the model
Check the switch circuit supply, signal and ground with a multimeter against the wiring diagram
Wiggle-test the harness and connector with the live data displayed to find intermittent faults
Test the switch for continuity and correct operation in each selector position
Check for software updates or technical bulletins for the vehicle if the circuit and switch test good

How to fix

Adjust the selector linkage or cable and set the switch position to specification
Replace the park/neutral position switch or transmission range sensor if it fails testing
Repair or replace damaged wiring in the signal, supply or ground circuit
Clean, dry and secure the connector, or replace it if the terminals are corroded or damaged
Repair the control module only after the wiring and switch are proven good, and apply any relevant manufacturer calibration update where an outdated calibration is a known cause

Can I keep driving?

Medium severity

The car may be driveable if it starts, but a no-start or poor gear engagement can occur without warning. Get it checked soon, and avoid relying on it if it is starting unreliably.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.