P0851
Park/Neutral Switch Input Circuit Low
- Severity
- High
- Driving
- Drivable — book it in
The signal from the park/neutral switch is stuck low - typically shorted to earth or a switch stuck closed. The module gets the same answer whatever the selector is doing, so it cannot tell which position has been chosen. Starting and the shift interlock are the first things affected.
Possible causes
- CommonWiring shorted to earth or a switch stuck closed between the switch and the module
- CommonFailed switch or transmission range sensor
- CommonCorroded or water-filled connector at the transmission
- PossibleSwitch out of adjustment so it never changes state
- PossibleChafed harness under the vehicle
What you might notice
- Engine will not crank at all, or cranks in any position
- Gear display wrong
- Reversing lamps wrong
- Transmission behaving as though the wrong gear were selected
- Check engine light on
What to check, in order
- Watch the switch state through the full selector travel. A reading that never changes confirms the fault.
- Unplug at the transmission and check whether the low reading persists at the harness.
- Check the switch or range sensor adjustment before condemning it.
- Inspect the connector for corrosion and water - it sits low and takes constant road spray.
- If the engine cranks with a gear selected, stop using the vehicle until this is repaired.
Read this code from your own car
Plug an ELM327 adapter into the OBD-II port under the dashboard. Pin16 reads every stored, pending and permanent code, explains each one like this page does — offline, with no account — and shows what the car recorded when the fault was set.
Where this code sits
P00xxP01xxP02xxP03xxP04xxP05xxP06xxP07xxP08xxP09xxP0AxxP0BxxP0CxxP0DxxP0ExxP0FxxP10xxP11xxP12xxP13xxP14xxP15xxP16xxP17xxP18xxP19xxP1AxxP1BxxP1CxxP1DxxP1ExxP1FxxP20xxP21xxP22xxP23xxP24xxP25xxP26xxP27xxP28xxP29xxP2AxxP2BxxP2CxxP2DxxP2ExxP2FxxP30xxP31xxP32xxP33xxP34xxP35xxP36xxP37xxP38xxP39xxP3AxxP3BxxP3CxxP3DxxP3ExxP3Fxx
Transmission
P0800P0801P0802P0803P0804P0805P0806P0807P0808P0809P0810P0811P0812P0813P0814P0815P0816P0817P0818P0819P0820P0821P0822P0823P0824P0825P0826P0827P0828P0829P0830P0831P0832P0833P0834P0835P0836P0837P0838P0839P0840P0841P0842P0843P0844P0845P0846P0847P0848P0849P0850P0851P0852P0853P0854P0855P0856P0857P0858P0859P0860P0861P0862P0863P0864P0865P0866P0867P0868P0869P0870P0871P0872P0873P0874P0875P0876P0877P0878P0879P0880P0881P0882P0883P0884P0885P0886P0887P0888P0889P0890P0891P0892P0893P0894P0895P0896P0897P0898P0899