Subaru Legacy IV (2008 year). Manual - part 565

EN(H6DO)(diag)-233

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

ENGINE (DIAGNOSTICS)

Step

Check

Yes

No

1

CHECK CURRENT DATA.
1) Start the engine.
2) Read data of intake manifold absolute pres-
sure signal using Subaru Select Monitor or gen-
eral scan tool.

NOTE:
• SUBARU SELECT MONITOR
For detailed operation procedure, refer to
“READ CURRENT DATA FOR ENGINE”. <Ref.
to EN(H6DO)(diag)-34, Subaru Select Moni-
tor.>
• General Scan Tool
For detailed operation procedures, refer to the
general scan tool operation manual.

Is the measured value 53.3 kPa 
(400 mmHg, 15.75 inHg) or 
more?

Make sure that the 
EGR valve, mani-
fold absolute pres-
sure sensor and 
throttle body are 
installed securely.

Go to step 2.

2

CHECK EGR VALVE.
Remove the EGR valve.

Are there holes, plugged piping 
or foreign objects caught in the 
EGR system?

Repair the EGR 
system.

Replace the EGR 
valve. <Ref. to 
FU(H6DO)-27, 
EGR Valve.>

EN(H6DO)(diag)-234

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

ENGINE (DIAGNOSTICS)

CC:DTC P0420 CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 1)

DTC DETECTING CONDITION:
• Two consecutive driving cycles with fault
• GENERAL DESCRIPTION <Ref. to GD(H6DO)-106, DTC P0420 CATALYST SYSTEM EFFICIENCY BE-
LOW THRESHOLD (BANK 1), Diagnostic Trouble Code (DTC) Detecting Criteria.>
TROUBLE SYMPTOM:
• Engine stalls.
• Idle mixture is out of specifications.

CAUTION:
After repair or replacement of faulty parts, perform Clear Memory Mode <Ref. to EN(H6DO)(diag)-52,
OPERATION, Clear Memory Mode.>, and Inspection Mode <Ref. to EN(H6DO)(diag)-44, PROCEDURE,
Inspection Mode.>.
WIRING DIAGRAM:

B47

3

4

5

6

1

2

B22

1 2 3 4
5 6 7 8
9 10 11 12

13 14 15 16

1 2 3 4
5 6 7 8

B83

B138

3 4

1 2

E25

E61

B47

1

2

3

5

4

6

E3

B22

14

MAIN RELAY

2

4

3

1

E25

13

15

16

E3

B22

10

11

12

9

SBF-5

BATTERY

2

E61

4

3

1

*

1

*

1

*

1

B83

B138

ECM

B134

A:

B135

B:

B136

C:

B137

D3

A3

D2

D1

D7

A5

D:

C5

*

*

2

2

C4

E

B1

B15

B4

B30

35

34

52

36

37

B21

E2

E

E

 : TERMINAL No. OPTIONAL ARRANGEMENT

*

1

 : TERMINAL No. OPTIONAL ARRANGEMENT 
  AMONG 1, 2, 5 AND 6

*

2

EN-06868

16

10 11 12 13 14 15

25

24

30

9

8

7

17 18 19 20
28

21 22 23

29

32

31

1

2

3

4

5

6

27

26

33 34 35

B136

C:

5

6

7

8

2

1

9

4

3

10

22 23

11 12 13 14 15

24 25

26

16 17

18 19 20 21

27

28 29

30 31

B137

D:

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

B135

B:

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

B134

A:

B21

1 2 3 4 5 6 7 8 9 10 11

12 13 14 15 16 17 18 19 20 21 22
23 24 25 26 27 28 29 30 31 32 33

34

35

42

43

36

37

38

39

48

49

50

51

52

53

54

40

41

44

45

46

47

REAR
OXYGEN 
SENSOR LH

REAR
OXYGEN 
SENSOR RH

EN(H6DO)(diag)-235

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

ENGINE (DIAGNOSTICS)

Step

Check

Yes

No

1

CHECK EXHAUST SYSTEM.
Check for gas leaks or air suction caused by 
loose or dislocated nuts and bolts, and open 
hole at exhaust pipes.

NOTE:
Check the following positions.
• Between cylinder head and front exhaust
pipe
• Between front exhaust pipe and front catalytic
converter
• Between front catalytic converter and rear
catalytic converter
• Looseness and improper attachment of front
oxygen (A/F) sensor or rear oxygen sensor

Is there any fault in exhaust 
system?

Repair or replace 
the exhaust sys-
tem. <Ref. to 
EX(H6DO)-2, Gen-
eral Description.>

Go to step 2.

2

CHECK WAVEFORM DATA ON THE SUBA-
RU SELECT MONITOR (WHILE DRIVING).
1) Drive at a constant speed between 80 — 
112 km/h (50 — 70 MPH).
2) After 5 minutes have elapsed in the condi-
tion of step 1), use the Subaru Select Monitor 
while still driving to read the waveform data.
• At normal condition

• At abnormal condition (numerous inversion)

Is normal waveform pattern dis-
played?

Even if DTC is 
detected, the cir-
cuit has returned to 
a normal condition 
at this time. Repro-
duce the failure, 
and then perform 
the diagnosis 
again.

NOTE:
In this case, tem-
porary poor con-
tact of connector
may be the cause.

Go to step 3.

EN-06666

1

0

1.5

0.5

10 sec/div

REAR OXYGEN 
SENSOR VOLTAGE 
                           (V)

A/F SENSOR 
OUTPUT LAMBDA 1

EN-06667

1

0

1.5

0.5

10 sec/div

REAR OXYGEN 
SENSOR VOLTAGE 
                           (V)

A/F SENSOR 
OUTPUT LAMBDA 1

EN(H6DO)(diag)-236

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

ENGINE (DIAGNOSTICS)

3

CHECK WAVEFORM DATA ON THE SUBA-
RU SELECT MONITOR (WHILE IDLING).
1) Run the engine at idle.
2) In the condition of step 1), use the Subaru 
Select Monitor to read the waveform data.
• At normal condition

• At abnormal condition 1 (numerous inversion)

• At abnormal condition 2 (noise input)

Is normal waveform pattern dis-
played?

Go to step 4.

• The waveform is 
displayed at abnor-
mal condition 1:Go 
to step 4.
• The waveform is 
displayed at abnor-
mal condition 2:Go 
to step 5.

4

CHECK CATALYTIC CONVERTER.

Is the catalytic converter dam-
aged?

Replace the cata-
lytic converter. 
<Ref. to 
EC(H6DO)-3, 
Front Catalytic 
Converter.>

Go to step 5.

5

CHECK REAR OXYGEN SENSOR CONNEC-
TOR AND COUPLING CONNECTOR.

Has water entered the connec-
tor?

Completely 
remove any water 
inside.

Go to step 6.

Step

Check

Yes

No

EN-06668

1

0

10 sec/div

REAR OXYGEN 
SENSOR VOLTAGE 
                           (V)

EN-06669

1

0

10 sec/div

REAR OXYGEN 
SENSOR VOLTAGE 
                           (V)

EN-06670

1

0

10 sec/div

REAR OXYGEN 
SENSOR VOLTAGE 
                           (V)