Subaru Legacy III (2000-2003 year). Service 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.
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
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
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
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
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.
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:
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