Powertrain, generic · Transmission

P0851 Park/Neutral Switch Input Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

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

Symptoms

  • MIL on (GM diesel, Subaru after two trips)
  • Cruise control not working
  • Rough idle when stopping

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€60–400
Time0.5–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

park neutral switchtransmission range sensorswitch wiringengine control module

Related codes

P0850P0852P0705P0706P084F

References: saabwisonline.com, saabwisonline.com, gsi.ext.gm.com, saabwisonline.com.