P0851Park/Neutral Switch Input Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- The park/neutral input tells the engine control module whether the automatic is in P or N, or a manual gearbox in neutral. It releases the starter, prepares idle control for the load of an engaged gear and enables cruise control and other monitors. All documented makers check it for plausibility against driving state or a second source such as the gearbox module. On GM's 6T70 and Subaru the switch pulls the line to ground in P/N. P0851 means the input wrongly reports P/N while the car drives under load. Subaru's manual sets it when the neutral switch stays on while the car goes from idle at standstill to at least 64 km/h three times, MIL after two trips, cruise control blocked. GM's LML Duramax compares it with the gearbox module above 24 km/h and 120 Nm, type B; GM petrol engines use type C.
- What you may notice
- MIL on (GM diesel, Subaru after two trips)
- Cruise control not working
- Rough idle when stopping
- Likely causes
- LikelyInhibitor or neutral switch faulty or misadjusted, stays closed
- PossibleSignal wire shorted to ground
- PossibleSelector cable misadjusted
- RareGearbox module status wrong where it is the reference
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €60 – €400
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.