Subaru Legacy (2005 year). Service manual - part 144

EN(H4SO 2.0)(diag)-165

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

AZ:DTC P0851 PARK/NEUTRAL SWITCH INPUT CIRCUIT LOW

1. AT MODEL

DTC DETECTING CONDITION:
Detects when malfunction occurs in 2 continuous driving cycles.
TROUBLE SYMPTOM:
Erroneous idling

CAUTION:
After repair or replacement of faulty parts, perform Clear Memory Mode <Ref. to EN(H4SO 2.0)(diag)-
38, OPERATION, Clear Memory Mode.> and Inspection Mode <Ref. to EN(H4SO 2.0)(diag)-32, PRO-
CEDURE, Inspection Mode.>.
WIRING DIAGRAM:

B12

T3

B14

T7

INHIBITOR SWITCH

NEUTRAL
POSITION
SWITCH

STARTER MOTOR

P

R

N

D

3

2

1

7

12

11

12

B135

ECM

12

B12

T7

1 2 3 4 5 6
7 8 9 10 11 12

1 2 3 4
5 6 7 8
9 10 11 12

E

1

B25

T2

T2

B25

2

MT

AT

EN-03485

M

36

B135

5

6

7

8

2

1

9

4

3

10

24

22 23

25

11 12 13 14 15

26 27

28

16 17 18 19

20 21

29 30 31

32 33

34 35

B21

1 2 3 4

12 13 14 15

5 6 7 8

16 17 18 19

9 10 11

20 21 22

23 24 25 26 27 28 29 30 31 32 33

35

34

37

36

39

38

41

40

43

42

44

45

47

46

49

48

51

50

53

52

54

B25

1 2

1 2 3 4
5 6 7 8

B83    : RHD

B122   : LHD

*

: TERMINAL No. RANDOM ARRANGEMENT

B83

B122 : LHD

: RHD

 E2

B21

*

*

EN(H4SO 2.0)(diag)-166

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

Step

Check

Yes

No

1

CHECK ANY OTHER DTC ON DISPLAY.

Is any other DTC displayed?

Check DTC using 
“List of Diagnostic 
Trouble Code 
(DTC)”. <Ref. to 
EN(H4SO 
2.0)(diag)-64, List 
of Diagnostic Trou-
ble Code (DTC).>

Go to step 2.

2

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

Connector & terminal

(B135) No. 12 (+) — Chassis ground (

−

):

Is the voltage more than 10 V? Even if the mal-

function indicator 
light illuminates, 
the circuit has 
returned to a nor-
mal condition at 
this time.

Go to step 3.

3

CHECK HARNESS BETWEEN ECM AND 
TRANSMISSION HARNESS CONNECTOR.
1) Turn the ignition switch to OFF.
2) Disconnect the connectors from ECM and 
transmission harness connector (T3).
3) Measure the resistance of harness 
between ECM connector and chassis ground.

Connector & terminal

(B135) No. 12 — Chassis ground:

Is the resistance more than 1 
M

Ω?

Go to step 4.

Repair the ground 
short circuit of har-
ness between 
ECM and trans-
mission harness 
connector.

4

CHECK TRANSMISSION HARNESS CON-
NECTOR.
1) Disconnect the connector from inhibitor 
switch.
2) Measure the resistance of harness 
between transmission harness connector and 
engine ground.

Connector & terminal

(T3) No. 11 — Engine ground:

Is the resistance more than 1 
M

Ω?

Go to step 5.

Repair the ground 
short circuit of har-
ness between 
transmission har-
ness connector 
and inhibitor 
switch connector.

5

CHECK INHIBITOR SWITCH.
Measure the resistance between inhibitor 
switch connector receptacle’s terminals with 
select lever at other than “N” and “P” range.

Terminals

(T7) No. 7 — No. 12:

Is the resistance more than 1 
M

Ω?

Go to step 6.

Replace the inhibi-
tor switch. <Ref. to 
4AT-51, Inhibitor 
Switch.>

6

CHECK SELECT CABLE CONNECTION.

Is there any fault in select 
cable connection to inhibitor 
switch?

Repair the select 
cable connection. 
<Ref. to CS-29, 
INSPECTION, 
Select Cable.>

Replace the ECM. 
<Ref. to FU(H4SO 
2.0)-34, Engine 
Control Module 
(ECM).>

EN(H4SO 2.0)(diag)-167

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

2. MT MODEL

DTC DETECTING CONDITION:
Detects when malfunction occurs in 2 continuous driving cycles.
TROUBLE SYMPTOM:
Erroneous idling

CAUTION:
After repair or replacement of faulty parts, perform Clear Memory Mode <Ref. to EN(H4SO 2.0)(diag)-
38, OPERATION, Clear Memory Mode.> and Inspection Mode <Ref. to EN(H4SO 2.0)(diag)-32, PRO-
CEDURE, Inspection Mode.>.
WIRING DIAGRAM:

B12

T3

B14

T7

INHIBITOR SWITCH

NEUTRAL
POSITION
SWITCH

STARTER MOTOR

P

R

N

D

3

2

1

7

12

11

12

B135

ECM

12

B12

T7

1 2 3 4 5 6
7 8 9 10 11 12

1 2 3 4
5 6 7 8
9 10 11 12

E

1

B25

T2

T2

B25

2

MT

AT

EN-03485

M

36

B135

5

6

7

8

2

1

9

4

3

10

24

22 23

25

11 12 13 14 15

26 27

28

16 17 18 19

20 21

29 30 31

32 33

34 35

B21

1 2 3 4

12 13 14 15

5 6 7 8

16 17 18 19

9 10 11

20 21 22

23 24 25 26 27 28 29 30 31 32 33

35

34

37

36

39

38

41

40

43

42

44

45

47

46

49

48

51

50

53

52

54

B25

1 2

1 2 3 4
5 6 7 8

B83    : RHD

B122   : LHD

*

: TERMINAL No. RANDOM ARRANGEMENT

B83

B122 : LHD

: RHD

 E2

B21

*

*

EN(H4SO 2.0)(diag)-168

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

Step

Check

Yes

No

1

CHECK INPUT SIGNAL OF ECM.
1) Turn the ignition switch to ON.
2) Place the shift lever in neutral.
3) Measure the voltage between ECM and 
chassis ground.

Connector & terminal

(B135) No. 12 (+) — Chassis ground (

−

):

Is the voltage less than 1 V?

Go to step 2.

Go to step 4.

2

CHECK INPUT SIGNAL OF ECM.
1) Place the shift lever except in neutral.
2) Measure the voltage between ECM and 
chassis ground.

Connector & terminal

(B135) No. 12 (+) — Chassis ground (

−

):

Is the voltage more than 10 V? Go to step 3.

Go to step 4.

3

CHECK POOR CONTACT.
Check poor contact in ECM connector.

Is there poor contact in ECM 
connector?

Repair the poor 
contact in ECM 
connector.

Replace the ECM. 
<Ref. to FU(H4SO 
2.0)-34, Engine 
Control Module 
(ECM).>

4

CHECK NEUTRAL POSITION SWITCH.
1) Turn the ignition switch to OFF.
2) Disconnect the connector from transmis-
sion harness.
3) Place the shift lever in neutral.
4) Measure the resistance between transmis-
sion harness and connector terminals.

Connector & terminal

(T2) No. 1 — No. 2:

Is the resistance less than 1 
Ω?

Go to step 5.

Repair the short 
circuit in transmis-
sion harness, or 
replace the neu-
tral position switch.

5

CHECK NEUTRAL POSITION SWITCH.
1) Place the shift lever except in neutral.
2) Measure the resistance between transmis-
sion harness connector terminals.

Connector & terminal

(T2) No. 1 — No. 2:

Is the resistance more than 1 
M

Ω?

Go to step 6.

Repair the short 
circuit in transmis-
sion harness, or 
replace the neu-
tral position switch.

6

CHECK HARNESS BETWEEN ECM AND 
NEUTRAL POSITION SWITCH CONNEC-
TOR.
Measure the resistance between ECM and 
chassis ground.

Connector & terminal

(B135) No. 12 — Chassis ground:

Is the resistance more than 1 
M

Ω?

Go to step 7.

Repair the ground 
short circuit of har-
ness between 
ECM and trans-
mission harness 
connector.

7

CHECK HARNESS BETWEEN ECM AND 
NEUTRAL POSITION SWITCH CONNEC-
TOR.
1) Disconnect the connector from ECM.
2) Measure the resistance of harness 
between ECM and transmission harness con-
nector.

Connector & terminal

(B135) No. 12 — (B25) No. 2:

Is the resistance less than 1 
Ω?

Go to step 8.

Repair the open 
circuit of harness 
between ECM and 
transmission har-
ness connector.

8

CHECK HARNESS BETWEEN ECM AND 
NEUTRAL POSITION SWITCH CONNEC-
TOR.
Measure the resistance of harness between 
transmission harness connector and engine 
ground.

Connector & terminal

(B25) No. 1 — Engine ground:

Is the resistance less than 5 
Ω?

Go to step 9.

Repair the open 
circuit between 
transmission har-
ness connector 
and engine ground 
terminal.