Subaru Legacy III (2000-2003 year). Manual - part 120

EN(H4SO)-92

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

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

—

• DTC DETECTING CONDITION:

• Two consecutive driving cycles with fault

CAUTION:
After repair or replacement of faulty parts, conduct Clear Memory Mode<Ref. to EN(H4SO)-47, OPER-
ATION, Clear Memory Mode.> and Inspection Mode <Ref. to EN(H4SO)-40, OPERATION, Inspection 
Mode.> .
• WIRING DIAGRAM:

EN-01144

F44

B61

6

RHD

RHD

BATTERY

SBF-5

B47

MAIN RELAY

1

2

3

5

4

6

F44

1 2 3 4
5 6 7 8

C16

C5

4

8

B22

E3

ECM

C6

C17

C22

C13

B23

3

2

1

FRONT OXYGEN 

(A/F) SENSOR

B18

B135

B:

B136

C:

B22

1 2 3 4
5 6 7 8
9 10 11 12

13 14 15 16

B47

3

4

5

6

1

2

B136

5 6

7

2

1

9

8

4

3

23

21 22

24

10 11 12 13 14

25 26

15 16 17

18 19 20

B18

4

2 1

3

LHD

LHD

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

B135

EN(H4SO)-93

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

Step

Value

Yes

No

1

CHECK ANY OTHER DTC ON DISPLAY.
Does the Subaru Select Monitor or OBD-II 
general scan tool indicate DTC P0031 and 
P0037 at the same time?

Indicated.

Go to step 2.

Go to step 5.

2

CHECK POWER SUPPLY TO FRONT OXY-
GEN (A/F) SENSOR.
1) Turn ignition switch to OFF.
2) Disconnect connector from front oxygen (A/

F) sensor.

3) Turn ignition switch to ON.
4) Measure voltage between front oxygen (A/

F) sensor connector and engine ground.

Connector & terminal

(B18) No. 4 (+) — Engine ground (

−−−−

):

Does the measured value exceed the spec-
ified value?

10 V

Go to step 3.

Repair power sup-
ply line.

NOTE:
In this case, repair
the following:
• Open circuit in 
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 con-
nector

3

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

Connector & terminal

(B136) No. 5 — Chassis ground:
(B136) No. 16 — Chassis ground:

Is the measured value less than the specified 
value?

5 

Ω

Go to step 4.

Repair harness 
and connector.

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

4

CHECK CURRENT DATA.
1) Start engine
2) Read data of front oxygen (A/F) sensor 

heater current using Subaru Select Monitor 
or OBD-II general scan tool.
Is the measured value less than the speci-
fied value?

NOTE:

•Subaru Select Monitor
For detailed operation procedure, refer to the 
“READ CURRENT DATA FOR ENGINE”. 
<Ref. to EN(H4SO)-32, Subaru Select Moni-
tor.>  
•OBD-II scan tool
For detailed operation procedures, refer to the 
OBD-II General Scan Tool Instruction Manual.

0.2 A

Repair poor con-
tact in connector.

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

Go to step 6.

5

CHECK OUTPUT SIGNAL FROM ECM.
1) Start and idle the engine.
2) Measure voltage between ECM connector 

and chassis ground.

Connector & terminal

(B136) No. 6 (+) — Chassis ground (

−−−−

):

(B136) No. 17 (+) — Chassis ground (

−−−−

):

Is the measured value less than the speci-
fied value?

1.0 V

Go to step 7.

Go to step 6.

EN(H4SO)-94

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

6

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

Connector & terminal

(B136) No. 6 (+) — Chassis ground (

−−−−

):

(B136) No. 17 (+) — Chassis ground (

−−−−

):

Is the measured value less than the specified 
value shaking harness and connector of ECM 
while monitoring the value with voltage meter?

1.0 V

Repair poor con-
tact in ECM con-
nector.

Go to step 7.

7

CHECK FRONT OXYGEN (A/F) SENSOR.
1) Turn ignition switch to OFF.
2) Measure resistance between front oxygen 

(A/F) sensor connector terminals.

Terminals

No. 3— No. 4:

Is the measured value less than the speci-
fied value?

10 

Ω

Repair harness 
and connector.

NOTE:
In this case, repair
the following:
• Open or ground 
short circuit in har-
ness between 
front oxygen (A/F) 
sensor and ECM 
connector
• Poor contact in 
front oxygen (A/F) 
sensor connector
• Poor contact in 
ECM connector

Replace front oxy-
gen (A/F) sensor.  
<Ref. to 
FU(H4SO)-41, 
Front Oxygen (A/
F) Sensor.>

Step

Value

Yes

No

EN(H4SO)-95

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

MEMO: