Subaru Legacy III (2000-2003 year). Manual - part 474

EN(H4DOSTC)-66

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

17.Diagnostic Procedure with Diagnostic Trouble Code (DTC)

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

—

CAUTION:
After repair or replacement of faulty parts, conduct Clear Memory Mode <Ref. to EN(H4DOSTC)-35, 
OPERATION, Clear Memory Mode.> and Inspection Mode <Ref. to EN(H4DOSTC)-33, Inspection 
Mode.> .
• WIRING DIAGRAM:

EN-00933

BATTERY

MAIN  RELAY

SBF-5

B47

1

2

3

5

4

6

E

E

E3

3

8

9

4

5

19

29

18

ECM

B137

B22

2

4

1

3

B18

FRONT OXYGEN
(A/F) SENSOR

3 4

1 2

3

4

1

2

5

6

1 2 3 4
5 6 7 8
9 10 11 12

13 14 15 16

B22

B47

B18

B137

1

2

7

8

9

5

6

3

4

10 11 12

19 20 21
29 30 31

13 14 15 16 17

27 28

18

22 23 24 25 26

EN(H4DOSTC)-67

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 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

(B18) No. 2 (+) — Engine ground (

−−−−

):

Does the measured value exceed the spec-
ified value?

10 V

Go to step 3.

Repair the power 
supply 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 the resistance of harness between 
ECM connector and chassis ground.

Connector & terminal

(B137) No. 8 — Chassis ground:
(B137) No. 9 — Chassis ground:

Is the measured value less than the specified 
value?

5 

Ω

Go to step 4.

Repair the har-
ness and connec-
tor.

NOTE:
In this case, repair
the following:
• Open circuit in 
harness between 
ECM and engine 
ground cable
• 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.
Is the measured value less than the speci-
fied value?

NOTE:

For detailed operation procedure, refer to the
“READ CURRENT DATA FOR ENGINE”. <Ref.
to EN(H4DOSTC)-26, Subaru Select Monitor.>  

0.2 A

Repair the poor 
contact in connec-
tor.

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

Go to step 5.

5

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

nector and chassis ground.

Connector & terminal

(B137) No. 4 (+) — Chassis ground (

−−−−

):

(B137) No. 5 (+) — Chassis ground (

−−−−

):

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

1.0 V

Go to step 7.

Go to step 6.

EN(H4DOSTC)-68

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

6

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

Connector & terminal

(B137) No. 4 (+) — Chassis ground (

−−−−

):

(B137) No. 5 (+) — 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 the poor 
contact in ECM 
connector.

Go to step 7.

7

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

gen (A/F) sensor connector terminals.

Terminals

No. 2 — No. 1:

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

10 

Ω

Repair the har-
ness and connec-
tor.

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 the front 
oxygen (A/F) sen-
sor.  <Ref. to 
FU(H4DOSTC)-
38, Front Oxygen 
(A/F) Sensor.>

Step

Value

Yes

No

EN(H4DOSTC)-69

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

MEMO: