Subaru Legacy IV (2008 year). Manual - part 545

EN(H6DO)(diag)-153

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

ENGINE (DIAGNOSTICS)

Step

Check

Yes

No

1

CHECK ENGINE COOLANT TEMPERATURE 
SENSOR.
Measure the resistance between engine cool-
ant temperature sensor terminals when the 
engine coolant is cold and after warmed up.

Terminals

No. 1 — No. 2:

Is the resistance of engine cool-
ant temperature sensor differ-
ent between when engine 
coolant is cold and after 
warmed up?

Repair the poor 
contact of ECM 
connector.

Replace the 
engine coolant 
temperature sen-
sor. <Ref. to 
FU(H6DO)-20, 
Engine Coolant 
Temperature Sen-
sor.>

EN(H6DO)(diag)-154

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

ENGINE (DIAGNOSTICS)

AJ:DTC P0128 COOLANT THERMOSTAT (ENGINE COOLANT TEMPERATURE 

BELOW THERMOSTAT REGULATING TEMPERATURE)

DTC DETECTING CONDITION:
• Two consecutive driving cycles with fault
• GENERAL DESCRIPTION <Ref. to GD(H6DO)-49, DTC P0128 COOLANT THERMOSTAT (ENGINE
COOLANT TEMPERATURE BELOW THERMOSTAT REGULATING TEMPERATURE), Diagnostic Trouble
Code (DTC) Detecting Criteria.>
TROUBLE SYMPTOM:
Thermostat remains open.

CAUTION:
After repair or replacement of faulty parts, perform Clear Memory Mode <Ref. to EN(H6DO)(diag)-52,
OPERATION, Clear Memory Mode.>, and Inspection Mode <Ref. to EN(H6DO)(diag)-44, PROCEDURE,
Inspection Mode.>.

Step

Check

Yes

No

1

CHECK ENGINE COOLANT.

Are the coolant level and mix-
ture ratio of engine coolant to 
anti-freeze solution correct?

Go to step 2.

Replace the 
engine coolant. 
<Ref. to 
CO(H6DO)-10, 
REPLACEMENT, 
Engine Coolant.>

2

CHECK RADIATOR FAN.
1) Start the engine.
2) Check the radiator fan operation.

Does the radiator fan continu-
ously rotate for 3 minutes or 
more during idling?

Repair radiator fan 
circuit. <Ref. to 
CO(H6DO)-19,
Radiator Main Fan 
and Fan Motor.> 
and <Ref. to 
CO(H6DO)-22,
Radiator Sub Fan 
and Fan Motor.>

Replace the ther-
mostat. <Ref. to 
CO(H6DO)-13, 
Thermostat.>

EN(H6DO)(diag)-155

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

ENGINE (DIAGNOSTICS)

AK:DTC P0131 O2 SENSOR CIRCUIT LOW VOLTAGE (BANK 1 SENSOR 1)

DTC DETECTING CONDITION:
• Immediately at fault recognition
• GENERAL DESCRIPTION <Ref. to GD(H6DO)-51, DTC P0131 O2 SENSOR CIRCUIT LOW VOLTAGE
(BANK 1 SENSOR 1), Diagnostic Trouble Code (DTC) Detecting Criteria.>

CAUTION:
After repair or replacement of faulty parts, perform Clear Memory Mode <Ref. to EN(H6DO)(diag)-52,
OPERATION, Clear Memory Mode.>, and Inspection Mode <Ref. to EN(H6DO)(diag)-44, PROCEDURE,
Inspection Mode.>.
WIRING DIAGRAM:

SBF-5

BATTERY

E

E3

B22

B47

1

2

3

5

4

6

MAIN RELAY

E3

B22

8

 : TERMINAL No. OPTIONAL ARRANGEMENT 
  AMONG 1, 2, 5 AND 6

*

3

4

5

6

1

2

B47

1 2 3 4
5 6 7 8
9 10 11 12

13 14 15 16

B22

3 4

1 2

E47

E24

16

10 11 12 13 14 15

25

24

30

9

8

7

17 18 19 20
28

21 22 23

29

32

31

1

2

3

4

5

6

27

26

33 34 35

B136

C:

5

6

7

8

2

1

9

4

3

10

22 23

11 12 13 14 15

24 25

26

16 17

18 19 20 21

27

28 29

30 31

B137

D:

1 2 3 4
5 6 7 8

B138

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

B135

B:

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

33 34

29

32

30 31

B134

A:

B11

2

E24

FRONT 

OXYGEN (A/F) 

SENSOR LH

3

4

B10

1

B7

B6

C2

C3

B8

4

1

3

B9

ECM

B135

B:

B134

A:

B137

D:

B136

C:

D3

A3

D2

D1

D7

A5

35

34

52

36

37

B21

E2

E

E

EN-06867

B21

1 2 3 4 5 6 7 8 9 10 11

12 13 14 15 16 17 18 19 20 21 22
23 24 25 26 27 28 29 30 31 32 33

34

35

42

43

36

37

38

39

48

49

50

51

52

53

54

40

41

44

45

46

47

2

E47

FRONT 

OXYGEN (A/F) 

SENSOR RH

5

6

7

4

2

1

3

B138

*

*

B1

EN(H6DO)(diag)-156

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

ENGINE (DIAGNOSTICS)

Step

Check

Yes

No

1

CHECK FRONT OXYGEN (A/F) SENSOR 
CONNECTOR AND COUPLING CONNEC-
TOR.

Has water entered the connec-
tor?

Completely 
remove any water 
inside.

Go to step 2.

2

CHECK HARNESS BETWEEN ECM AND 
FRONT OXYGEN (A/F) SENSOR CONNEC-
TOR.
1) Turn the ignition switch to OFF.
2) Disconnect the connectors from ECM and 
front oxygen (A/F) sensor connector.
3) Measure the resistance between ECM and 
chassis ground.

Connector & terminal

(B135) No. 9 — Chassis ground:
(B135) No. 8 — Chassis ground:

Is the resistance 1 M

: or 

more?

Go to step 3.

Repair the ground 
short circuit of har-
ness between 
ECM and front oxy-
gen (A/F) sensor 
connector.

3

CHECK POOR CONTACT.
Check for poor contact in the front oxygen (A/F) 
sensor connector.

Is there poor contact in front 
oxygen (A/F) sensor connec-
tor?

Repair the poor 
contact of the front 
oxygen (A/F) sen-
sor connector.

Replace the front 
oxygen (A/F) sen-
sor. <Ref. to 
FU(H6DO)-32, 
Front Oxygen (A/F) 
Sensor.>