Subaru Legacy (2005 year). Manual - part 120

EN(H4SO 2.0)(diag)-69

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

18.Diagnostic Procedure with Diagnostic Trouble Code (DTC)

A: DTC P0030 HO2S HEATER CONTROL CIRCUIT (BANK 1 SENSOR 1)

DTC DETECTING CONDITION:
Detects when malfunction occurs in 2 continuous driving cycles.

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:

EN-03475

3

4

1

2

5

6

B47

SBF-5

E

B47

E24

5
3

4
6

2
1

C34

C35

C33

B3

B2

B1

B135

E

B5

B6

52

B21

E2

B21

E2

50

51

3

4

E2

B21

E

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

E24

ECM

BATTERY

FRONT OXYGEN

(A/F) SENSOR

MAIN RELAY

B136

B:

C:

1

2

3

4

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

B:

B136

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

35

C:

E24

3

2

*

1

*

*

3

  LHD : 4

RHD : 1

*

1

*

*

2

3

3

1

6 5 4

2

: RHD

: LHD

LHD : 1

RHD : 4

LHD : 2

RHD : 6

EN(H4SO 2.0)(diag)-70

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

Step

Check

Yes

No

1

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

Connector & terminal

(B135) No. 2 — (E24) No. 3:
(B135) No. 3 — (E24) No. 3:

Is the resistance less than 1 
Ω?

Go to step 2.

Repair the open 
circuit of harness 
between ECM and 
front oxygen (A/F) 
sensor connector.

2

CHECK HARNESS BETWEEN ECM AND 
FRONT OXYGEN (A/F) SENSOR CONNEC-
TOR.
Measure the resistance of harness between 
ECM and front oxygen (A/F) sensor connector.

Connector & terminal
LHD model

(B136) No. 33 — (E24) No. 1:
(B136) No. 35 — (E24) No. 2:

RHD model

(B136) No. 33 — (E24) No. 4:
(B136) No. 35 — (E24) No. 6:

Is the resistance less than 1 
Ω?

Go to step 3.

Repair the open 
circuit of harness 
between ECM and 
front oxygen (A/F) 
sensor connector.

3

CHECK HARNESS BETWEEN MAIN RELAY 
AND FRONT OXYGEN (A/F) SENSOR CON-
NECTOR.
Measure the resistance of harness between 
main relay and front oxygen (A/F) sensor con-
nector.

Connector & terminal

LHD model

(B47) No. 3 — (E24) No. 4:

RHD model

(B47) No. 3 — (E24) No. 1:

Is the resistance less than 1 
Ω?

Go to step 4.

Repair the open 
circuit of harness 
between ECM and 
front oxygen (A/F) 
sensor connector.

4

CHECK FRONT OXYGEN (A/F) SENSOR.
Measure the resistance between front oxygen 
(A/F) sensor connector terminals.

Terminals

LHD model

No. 3 — No. 4:

RHD model

No. 1 — No. 3:

Is the resistance less than 5 
Ω?

Go to step 5.

Replace the front 
oxygen (A/F) sen-
sor. <Ref. to 
FU(H4SO 2.0)-32, 
Front Oxygen (A/
F) Sensor.>

5

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

Is there poor contact in ECM or 
front oxygen (A/F) sensor con-
nector?

Repair the poor 
contact in ECM or 
front oxygen (A/F) 
sensor connector.

Replace the front 
oxygen (A/F) sen-
sor. <Ref. to 
FU(H4SO 2.0)-32, 
Front Oxygen (A/
F) Sensor.>

EN(H4SO 2.0)(diag)-71

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

B: DTC P0031 HO2S HEATER CONTROL CIRCUIT LOW (BANK 1 SENSOR 1)

DTC DETECTING CONDITION:
Immediately at fault recognition

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:

Step

Check

Yes

No

1

CHECK ANY OTHER DTC ON DISPLAY.

Do DTC P0031 and P0037 dis-
play at the same time on the 
Subaru Select Monitor?

Go to step 2.

Go to step 5.

EN-03475

3

4

1

2

5

6

B47

SBF-5

E

B47

E24

5
3

4
6

2
1

C34

C35

C33

B3

B2

B1

B135

E

B5

B6

52

B21

E2

B21

E2

50

51

3

4

E2

B21

E

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

E24

ECM

BATTERY

FRONT OXYGEN

(A/F) SENSOR

MAIN RELAY

B136

B:

C:

1

2

3

4

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

B:

B136

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

35

C:

E24

3

2

*

1

*

*

3

  LHD : 4

RHD : 1

*

1

*

*

2

3

3

1

6 5 4

2

: RHD

: LHD

LHD : 1

RHD : 4

LHD : 2

RHD : 6

EN(H4SO 2.0)(diag)-72

ENGINE (DIAGNOSTICS)

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

2

CHECK POWER SUPPLY TO FRONT OXY-
GEN (A/F) SENSOR.
1) Turn the ignition switch to OFF.
2) Disconnect the connector from front oxygen 
(A/F) sensor.
3) Turn the ignition switch to ON.
4) Measure the voltage between front oxygen 
(A/F) sensor connector and engine ground.

Connector & terminal

LHD model

(E24) No. 4 (+) — Engine ground (

−

):

RHD model

(E24) No. 1 (+) — Engine ground (

−

):

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

Repair the power 
supply line.

NOTE:
In this case, repair
the following item:
• Open circuit of 
harness between 
main relay and 
front oxygen (A/F) 
sensor connector
• Poor contact in 
front oxygen (A/F) 
sensor connector
• Poor contact in 
main relay connec-
tor

3

CHECK GROUND CIRCUIT FOR ECM.
Measure the resistance of harness between 
ECM connector and chassis ground.

Connector & terminal

(B135) No. 5 — Chassis ground:
(B135) No. 6 — Chassis ground:

Is the resistance less than 5 
Ω?

Go to step 4.

Repair the har-
ness and connec-
tor.

NOTE:
In this case, repair
the following item:
• Open circuit of 
harness between 
ECM and engine 
ground terminal
• Poor contact in 
ECM connector
• Poor contact in 
coupling connector

4

CHECK CURRENT DATA.
1) Start the engine.
2) Read the data of front oxygen (A/F) sensor 
heater current using Subaru Select Monitor or 
general scan tool.

NOTE:

• Subaru Select Monitor
For detailed operation procedure, refer to 
“READ CURRENT DATA FOR ENGINE”. <Ref. 
to EN(H4SO 2.0)(diag)-25, Subaru Select 
Monitor.>
• General scan tool
For detailed operation procedure, refer to the 
general scan tool operation manual.

Is the current more than 0.2 A? Repair the poor 

contact in connec-
tor.

NOTE:
In this case, repair
the following item:
• Poor contact in 
front oxygen (A/F) 
sensor connector
• Poor contact in 
ECM connector

Go to step 5.

5

CHECK INPUT SIGNAL OF ECM.
1) Start and idle the engine.
2) Measure the voltage between ECM con-
nector and chassis ground.

Connector & terminal

(B135) No. 2 (+) — Chassis ground (

−

):

(B135) No. 3 (+) — Chassis ground (

−

):

Is the voltage less than 1 V?

Go to step 7.

Go to step 6.

6

CHECK OUTPUT SIGNAL FROM ECM.
Measure the voltage between ECM connector 
and chassis ground.

Connector & terminal

(B135) No. 2 (+) — Chassis ground (

−

):

(B135) No. 3 (+) — Chassis ground (

−

):

Does the voltage change by 
shaking the ECM harness and 
connector while monitoring the 
value of voltage meter?

Repair the poor 
contact in ECM 
connector.

Go to step 7.

Step

Check

Yes

No