Subaru Legacy III (2000-2003 year). Manual - part 485

EN(H4DOSTC)-110

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

O: DTC P0136 — O2 SENSOR CIRCUIT (BANK 1 SENSOR 2) —

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-00934

BATTERY

MAIN  RELAY

B47

1

2

3

5

4

6

E

E

E3

3

3
6

B22

B122

3

4

1

2

5

6

1 2 3 4
5 6 7 8
9 10 11 12

13 14 15 16

B22

B135

B136

B122

B47

T5

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

1

4

2

3

T6

         REAR
OXYGEN SENSOR

D8

D9

C13

B17

B26

B19

ECM

B136

B137

C :

B135

B :

D :

1 2 3 4 5 6

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

1 2

7

8 9

5

6

3

4

10 11 12
19 20 21

13 14 15 16

17 18

22 23 24

1

2

3

4

2

4

3

1

T5

B19

T5

B19

T6

2 1
4 3

EN(H4DOSTC)-111

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

Step

Value

Yes

No

1

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 the data of rear oxygen sensor signal 

using Subaru Select Monitor.
Does the value fluctuate?

NOTE:

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

The value fluctuates.

Go to step 5.

Go to step 2.

2

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

0.2 — 0.4 V

Go to step 3.

Replace the rear 
oxygen sensor.  
<Ref. to 
FU(H4DOSTC)-
39, Rear Oxygen 
Sensor.>

3

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

rear oxygen sensor.

3) Measure the resistance of harness 

between ECM and rear oxygen sensor con-
nector.

Connector & terminal

(B135) No. 26 — (T6) No. 4:

Does the measured value exceed the spec-
ified value?

3 

Ω

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

Go to step 4.

4

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

sensor.

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

sensor harness connector and engine 
ground or chassis ground.

Connector & terminal

(T6) No. 4 (+) — Engine ground (

−−−−

):

Does the measured value exceed the spec-
ified value?

0.2 V

Replace the rear 
oxygen sensor.  
<Ref. to 
FU(H4DOSTC)-
39, Rear Oxygen 
Sensor.>

Repair the har-
ness and connec-
tor.

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

5

CHECK EXHAUST SYSTEM.
Check the exhaust system parts.
Is there a fault in exhaust system?

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

There is a fault.

Repair or replace 
the faulty parts.

Replace the rear 
oxygen sensor.  
<Ref. to 
FU(H4DOSTC)-
39, Rear Oxygen 
Sensor.>

EN(H4DOSTC)-112

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

P: DTC P0171 — SYSTEM TOO LEAN (BANK 1) —

NOTE:
For the diagnostic procedure, refer to DTC P0172. <Ref. to EN(H4DOSTC)-114, DTC P0172 — SYSTEM
TOO RICH (BANK 1) —, Diagnostic Procedure with Diagnostic Trouble Code (DTC).>

EN(H4DOSTC)-113

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

MEMO: