P0851 code meaning
Park/Neutral Switch Input Circuit Low
Powertrain system Generic code P08xx codes Check engine light
What P0851 means
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 gasoline engines use type C.
P0851 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 2 hours
- Clears after repairAfter a drive cycle
Symptoms of P0851
- MIL on (GM diesel, Subaru after two trips)
- Cruise control not working
- Rough idle when stopping
Common causes of P0851
- Most likelyInhibitor or neutral switch faulty or misadjusted, stays closed
- PossibleSignal wire shorted to ground
- PossibleSelector cable misadjusted
- Less likelyGearbox module status wrong where it is the reference
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Park neutral switchTransmission range sensorSwitch wiringEngine control module
What each part of P0851 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 8 | Area: transmission. |
| 51 | The exact fault in that area. |
What to do next
- Note P0851 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0851
What does P0851 mean?
P0851 means "Park/Neutral Switch Input Circuit Low". The park/neutral input tells the engine control module whether the automatic is in P or N, or a manual gearbox in neutral. It is a generic code, so it means the same on every make.
Is it safe to drive with P0851?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0851 fail an emissions test?
Yes, it can. P0851 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0851 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2 hours of work. This is a job for a workshop.
Will P0851 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.