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.
Medium severity. Can I keep driving?Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
Symptoms
- MIL on (GM diesel, Subaru after two trips)
- Cruise control not working
- Rough idle when stopping
Most likely causes
- Inhibitor or neutral switch faulty or misadjusted, stays closed
- Signal wire shorted to ground
- Selector cable misadjusted
- Gearbox module status wrong where it is the reference
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P0851 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 8: Transmission
- 51: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0851?
Drive gently to a repair shop. If it slips, you hear clunks, or it loses its gears, do not keep driving.
What causes code P0851?
The most common causes are: inhibitor or neutral switch faulty or misadjusted, stays closed; signal wire shorted to ground; selector cable misadjusted.
Related codes
- P0850Park/Neutral Switch Input Circuit
- P0852Park/Neutral Switch Input Circuit High
- P0705Transmission Range Sensor A Circuit (PRNDL Input)
- P0706Transmission Range Sensor A Circuit Range/Performance
- P084FPark/Neutral Switch Output Circuit
Sources
- saabwisonline.com: dtc p0851 neutral switch input circuit low mt mode
- saabwisonline.com: dtc p0851 neutral switch input circuit low at mode
- gsi.ext.gm.com: 14OBDG10 Engine Diagnostics
- saabwisonline.com: dtc p0850 p0852
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.