Subaru Legacy III (2000-2003 year). Manual - part 342

EN(H6DO)-170

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

Z: DTC P0138 — O2 SENSOR CIRCUIT HIGH VOLTAGE (BANK 1 SENSOR 2) —

• DTC DETECTING CONDITION:

• Two consecutive driving cycles with fault

CAUTION:
After repair or replacement of faulty parts, conduct Clear Memory Mode <Ref. to EN(H6DO)-54, OP-
ERATION, Clear Memory Mode.> and Inspection Mode <Ref. to EN(H6DO)-47, OPERATION, Inspec-
tion Mode.>.
• WIRING DIAGRAM:

EN-01088

BATTERY

F44

B61

6

T5

B19

B47

MAIN RELAY

2
1

3

5

4

6

F44

1 2 3 4
5 6 7 8

C13

B17

B19
B26

D21

D31

D9

D8

REAR 

OXYGEN 

SENSOR

5

3

6
4

1

3

4

2

1

5

B252

B83

B19

T5

T6

E49

B47

T6

B136

C:

D:

B135

B:

3

4

5

6

1

2

5

7

ECM

B135

B:

B136

C:

B137

D:

3

4

1

2

1 2

7

8 9

5

6

3

4

10 11 12
19 20 21

13 14 15 16

17 18

22 23 24

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

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

RHD

LHD

LHD

RHD

B252

1 2 3 4
5 6 7 8

B19

4

1 2 3

6

5

2

B83

1 2 3 4

5 6 7 8 9 10

11 12

19

20

13 14 15 16 17 18

EN(H6DO)-171

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

Step

Value

Yes

No

1

CHECK ANY OTHER DTC ON DISPLAY.
Is any other DTC is displayed?

Another DTC is displayed.

Check the relevant 
DTC using “List of 
Diagnostic Trou-
ble Code (DTC)”. 
<Ref. to 
EN(H6DO)-89, List 
of Diagnostic 
Trouble Code 
(DTC).>

Go to step 2.

2

CHECK REAR OXYGEN SENSOR DATA.
1) Warm-up the engine until engine coolant 

temperature is above 70

°

C (158

°

F), and 

keep the engine speed at 2,000 rpm to 
3,000 rpm for two minutes.

2) Read data of rear oxygen sensor signal 

using Subaru Select Monitor or OBD-II gen-
eral scan tool.
Does the value fluctuate?

NOTE:

• Subaru Select Monitor
For detailed operation procedure, refer to the 
“READ CURRENT DATA SHOWN ON DIS-
PLAY FOR ENGINE”. <Ref. to EN(H6DO)-34, 
Subaru Select Monitor.>
• OBD-II general scan tool
For detailed operation procedures, refer to the 
OBD-II General Scan Tool Instruction Manual.

Value fluctuates.

Go to step 6.

Go to step 3.

3

CHECK REAR OXYGEN SENSOR DATA.
Read data of rear oxygen sensor signal using 
Subaru Select Monitor or OBD-II General Scan 
Tool.
Is the measured value within the specified 
range?

Output maximum value 0.49 V 
or more and output minimum 
value 0.25 V or less. 

Go to step 4.

Replace rear oxy-
gen sensor. <Ref. 
to FU(H6DO)-45, 
Rear Oxygen Sen-
sor.> 

4

CHECK HARNESS BETWEEN ECM AND 
REAR OXYGEN SENSOR CONNECTOR.
1) Turn ignition switch to OFF.
2) Disconnect connectors from ECM and rear 

oxygen sensor.

3) Measure resistance of harness between 

ECM and rear oxygen sensor connector.

Connector & terminal

(B135) No. 19 — (T6) No. 4:

Does the measured value exceed the spec-
ified value?

3 

Ω

Repair open circuit 
in harness 
between ECM and 
rear oxygen sen-
sor connector.

Go to step 5.

5

CHECK HARNESS BETWEEN REAR OXY-
GEN SENSOR AND ECM CONNECTOR.
1) Turn ignition switch to OFF.
2) Disconnect connector from rear oxygen 

sensor.

3) Turn ignition switch to ON.
4) Measure voltage between rear oxygen sen-

sor harness connector and engine ground 
or chassis ground.

Connector & terminal

(T6) No. 3 (+) — Engine ground (

−−−−

):

Does the measured value exceed the spec-
ified value?

0.2 V

Replace rear oxy-
gen sensor. <Ref. 
to FU(H6DO)-45, 
Rear Oxygen Sen-
sor.>

Repair harness 
and connector.

NOTE:
In this case, repair
the following:
• Open circuit in 
harness between 
rear oxygen sen-
sor and ECM con-
nector
• Poor contact in 
rear oxygen sen-
sor connector
• Poor contact in 
ECM connector

EN(H6DO)-172

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

6

CHECK EXHAUST SYSTEM.
Check exhaust system parts.

NOTE:

Check the following items.
•  Loose installation of portions
• Damage (crack, hole etc.) of parts
• Looseness and ill fitting of parts between 
front oxygen (A/F) sensor and rear oxygen 
sensor
Is there a fault in exhaust system?

There is a trouble.

Repair or replace 
faulty parts.

Replace rear oxy-
gen sensor. <Ref. 
to FU(H6DO)-45, 
Rear Oxygen Sen-
sor.>

Step

Value

Yes

No

EN(H6DO)-173

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

MEMO: