Subaru Legacy BN/BS (2015-2019) Service Manual: Dtc p0851 park/neutral switch input circuit low

Subaru Legacy BN/BS (2015-2019) Service Manual / Engine (diagnostics)(h4do) / Diagnostic procedure with diagnostic trouble code (dtc) / Dtc p0851 park/neutral switch input circuit low

DTC DETECTING CONDITION:

Detected when two consecutive driving cycles with fault occur.

TROUBLE SYMPTOM:

Improper idling

CAUTION:

After servicing or replacing faulty parts, perform Clear Memory Mode Clear Memory Mode > OPERATION, and Inspection Mode Inspection Mode > PROCEDURE.

1. CVT MODEL

WIRING DIAGRAM:

Engine Electrical System Engine Electrical System > WIRING DIAGRAM

• Engine Electrical System ENGINE TYPE FB (WITHOUT PUSH BUTTON START) Engine Electrical System > WIRING DIAGRAM

• Engine Electrical System ENGINE TYPE FB (WITH PUSH BUTTON START) Engine Electrical System > WIRING DIAGRAM

STEP	CHECK	YES	NO
1.CHECK SELECT CABLE.	Is there any fault in select cable-	Repair or adjust the select cable. Select Cable	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC P0851 PARK/NEUTRAL SWITCH INPUT CIRCUIT LOW
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-	Repair the poor contact of ECM connector.	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC P0851 PARK/NEUTRAL SWITCH INPUT CIRCUIT LOW
3.CHECK HARNESS BETWEEN ECM AND TRANSMISSION HARNESS CONNECTOR. 1) Turn the ignition switch to OFF. 2) Disconnect the connectors from ECM and inhibitor switch. 3) Measure the resistance between ECM connector and chassis ground. Connector & terminal (B134) No. 5 — Chassis ground:	Is the resistance 1 M- or more-	Replace the inhibitor switch. Inhibitor Switch	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

2. OUTLINE OF DIAGNOSIS

Detect the open or short circuit of neutral SW.

Judge as NG when the ECM neutral terminal input differs from the reception data from TCM.

3. EXECUTION CONDITION

Secondary Parameters	Execution condition
Battery voltage	≥ 10.9 V
Starter relay feedback voltage	< Battery voltage - 0.35
Data received from TCM	≠ “P” range/“N” range

4. GENERAL DRIVING CYCLE

Perform the diagnosis only once after the enable conditions have been established.

5. DIAGNOSTIC METHOD

If the duration of time while the following conditions are met is longer than the time indicated, judge as NG.

Judgment Value

Malfunction Criteria	Threshold Value
Voltage of the neutral position switch signal	≤ Battery voltage - 0.19

Time Needed for Diagnosis: 64 ms - 100 time(s)

Malfunction Indicator Light Illumination: Illuminates when malfunction occurs in 2 continuous driving cycles.

6. MT MODEL

WIRING DIAGRAM:

• Engine Electrical System ENGINE TYPE FB (WITHOUT PUSH BUTTON START) Engine Electrical System > WIRING DIAGRAM

• Engine Electrical System ENGINE TYPE FB (WITH PUSH BUTTON START) Engine Electrical System > WIRING DIAGRAM

STEP	CHECK	YES	NO
1.CHECK INPUT SIGNAL OF ECM. 1) Turn the ignition switch to ON. 2) Place the shift lever in a position other than neutral. 3) Measure the voltage between ECM connector and chassis ground. Connector & terminal (B134) No. 5 (+) — Chassis ground (−):	Is the voltage 10 V or more-	Repair the poor contact of ECM connector.	Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC P0851 PARK/NEUTRAL SWITCH INPUT CIRCUIT LOW
2.CHECK HARNESS BETWEEN ECM AND NEUTRAL POSITION SWITCH CONNECTOR. 1) Turn the ignition switch to OFF. 2) Disconnect the connectors from ECM and neutral position switch. 3) Measure the resistance between ECM connector and chassis ground. Connector & terminal (B134) No. 5 — Chassis ground:	Is the resistance 1 M- or more-	Replace the neutral position switch. Switches and Harness	Repair the short circuit to ground harness between ECM connector and neutral position switch connector.

7. OUTLINE OF DIAGNOSIS

Detect the open or short circuit of neutral SW.

Judge as NG when there is no change in the neutral SW even if the driving shift was applied. (There is neutral SW ON/OFF inversion from the vehicle speed and engine speed.)

8. EXECUTION CONDITION

Secondary Parameters	Execution condition
Battery voltage	≥ 10.9 V
The number of driving condition change from a) to b) a) Vehicle speed & Engine speed	= 3 time(s) ≤ 0 km/h (0 MPH) ≥ 550 rpm and ≤ 900 rpm
b) Vehicle speed & Engine speed	≥ 64 km/h ≥ 1500 rpm and ≤ 2150 rpm

9. GENERAL DRIVING CYCLE

Perform the diagnosis continuously after the enable conditions have been established.

10. DIAGNOSTIC METHOD

Judge NG when the malfunction criteria below are completed determined times or more after the neutral SW change.

Judgment Value

Malfunction Criteria	Threshold Value
Neutral switch output voltage	≤ Battery voltage - 0.19 V

Time Needed for Diagnosis: 3 time(s)

Malfunction Indicator Light Illumination: Illuminates when malfunction occurs in 2 continuous driving cycles.

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