DTC P0851: Park/Neutral Switch Input Circuit Low: Notes

DTC detecting condition: 

Detected when two consecutive driving cycles with fault occur.

Trouble symptom: 

Improper idling

Wiring diagram: 

Engine Electrical System Ref. to WIRING SYSTEM>Engine Electrical System>WIRING DIAGRAM

Courtesy of SUBARU OF AMERICA, INC.

1. CHECK SELECT CABLE. 

   Check the select cable.

   Is there any fault in select cable?

   Yes:  Repair or adjust the select cable. Ref. to CONTROL SYSTEMS>SELECT CABLE

   No:  Go to  2

2. CHECK INPUT SIGNAL OF ECM. 
   1. Turn the ignition switch to ON.
   2. Place the select lever in other than "P" range and "N" range.
   3. Measure the voltage between ECM connector and chassis ground.

      Connector & terminal 

      (B134) No. 5 (+) - Chassis ground (-):

   Is the voltage 10 V or more?

   Yes:  Repair the poor contact of ECM connector.

   No:  Go to  3

3. CHECK HARNESS BETWEEN ECM AND TRANSMISSION HARNESS CONNECTOR. 
   1. Turn the ignition switch to OFF.
   2. Disconnect the connector from ECM.
   3. Disconnect the connector from inhibitor switch.
   4. Measure the resistance between ECM connector and chassis ground.

      Connector & terminal 

      (B134) No. 5 - Chassis ground:

   Is the resistance 1 MΩ or more?

   Yes:  Replace the inhibitor switch. Ref. to CONTINUOUSLY VARIABLE TRANSMISSION(TR580)>INHIBITOR SWITCH

   No:  Repair the harness and connector.

   NOTE: In this case, repair the following item:
   • Short circuit to ground in harness between ECM connector and inhibitor switch connector
   • Poor contact of coupling connector