Subaru Legacy III (2000-2003 year). Manual - part 497

EN(H4DOSTC)-158

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

AH:DTC P0508 — IDLE CONTROL SYSTEM CIRCUIT LOW —

• TROUBLE SYMPTOM:

• Erroneous idling
• Engine stalls.
• Engine breathing

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-00947

BATTERY

MAIN RELAY

E

E

6
4

5
3

2
1

10

       IDLE
AIR CONTROL
  SOLENOID
     VALVE

1

3

E7

B47

B47

3

4

1

2

5

6

1 2

7

8 9

5

6

3

4

10 11 12
19 20 21

13 14 15 16

17 18

22 23 24

B136

B136 ECM

2

1

B22

E3

SBF-5

B22

E7

1 2 3

1 2

5

6 7

8

13

14 15

16

9 10

11 12

3 4

17 18

19 20

1 2 3 4
5 6 7 8
9 10 11 12

13 14 15 16

B21

E2

B21

12

EN(H4DOSTC)-159

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

Step

Value

Yes

No

1

CHECK OUTPUT SIGNAL FROM ECM.
1) Turn the ignition switch to ON.
2) Measure the voltage between ECM and 

chassis ground.

Connector & terminal

(B136) No. 10 (+) — Chassis ground (

−−−−

):

Does the measured value exceed the spec-
ified value?

3 V

Repair the poor 
contact in ECM 
connector.

Go to step 2.

2

CHECK POWER SUPPLY TO IDLE AIR CON-
TROL SOLENOID VALVE.
1) Turn the ignition switch to OFF.
2) Disconnect the connector from idle air con-

trol solenoid valve.

3) Turn the ignition switch to ON.
4) Measure the voltage between idle air con-

trol solenoid valve and engine ground.

Connector & terminal

(E7) No. 2 (+) — Engine ground (

−−−−

):

Does the measured value exceed the spec-
ified value?

10 V

Go to step 3.

Repair the har-
ness and connec-
tor.

NOTE:
In this case, repair
the following:
• Open circuit in 
harness between 
idle air control 
solenoid valve and 
main relay con-
nector
• Poor contact in 
coupling connector

3

CHECK HARNESS BETWEEN ECM AND 
IDLE AIR CONTROL SOLENOID VALVE 
CONNECTOR.
1) Turn the ignition switch to OFF.
2) Disconnect the connector from ECM.
3) Measure the resistance of harness 

between ECM and idle air control solenoid 
valve connector.

Connector & terminal

(B136) No. 10 — (E7) No. 1:

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

1 

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 idle air 
control solenoid 
valve connector
• Poor contact in 
coupling connector

4

CHECK HARNESS BETWEEN ECM AND 
IDLE AIR CONTROL SOLENOID VALVE 
CONNECTOR.
Measure the resistance of harness between 
ECM and chassis ground.

Connector & terminal

(B136) No. 10 — Chassis ground:

Does the measured value exceed the specified 
value?

1 M

Ω

Go to step 5.

Repair the ground 
short circuit in har-
ness between 
ECM and idle air 
control solenoid 
valve connector.

5

CHECK GROUND CIRCUIT OF IDLE AIR 
CONTROL SOLENOID VALVE.
Measure the resistance of harness between 
idle air control solenoid valve connector and 
engine ground.

Connector & terminal

(E7) No. 3 — Engine ground:

Is the measured value less than the specified 
value?

5 

Ω

Go to step 6.

Repair the open 
circuit in harness 
between idle air 
control solenoid 
valve connector 
and engine ground 
cable.

EN(H4DOSTC)-160

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

6

CHECK POOR CONTACT.
Check poor contact in ECM and idle air control 
solenoid valve connectors. 
Is there poor contact in ECM and idle air con-
trol solenoid valve connectors?

There is poor contact.

Repair the poor 
contact in ECM 
and idle air control 
solenoid valve 
connectors.

Replace the idle 
air control solenoid 
valve. <Ref. to 
FU(H4DOSTC)-
35, Idle Air Con-
trol Solenoid 
Valve.>

Step

Value

Yes

No

EN(H4DOSTC)-161

ENGINE (DIAGNOSTICS)

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)

MEMO: