Subaru Legacy III (2000-2003 year). Service manual — part 120
EN(H4SO)-92
ENGINE (DIAGNOSTICS)
DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)
B: DTC P0031 — HO2S HEATER CONTROL CIRCUIT LOW (BANK 1 SENSOR 1)
—
• DTC DETECTING CONDITION:
• Two consecutive driving cycles with fault
CAUTION:
After repair or replacement of faulty parts, conduct Clear Memory Mode<Ref. to EN(H4SO)-47, OPER-
ATION, Clear Memory Mode.> and Inspection Mode <Ref. to EN(H4SO)-40, OPERATION, Inspection
Mode.> .
• WIRING DIAGRAM:
EN-01144
F44
B61
6
RHD
RHD
BATTERY
SBF-5
B47
MAIN RELAY
1
2
3
5
4
6
F44
1 2 3 4
5 6 7 8
C16
C5
4
8
B22
E3
ECM
C6
C17
C22
C13
B23
3
2
1
FRONT OXYGEN
(A/F) SENSOR
B18
B135
B:
B136
C:
B22
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16
B47
3
4
5
6
1
2
B136
5 6
7
2
1
9
8
4
3
23
21 22
24
10 11 12 13 14
25 26
15 16 17
18 19 20
B18
4
2 1
3
LHD
LHD
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
B135
EN(H4SO)-93
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 ignition switch to OFF.
2) Disconnect connector from front oxygen (A/
F) sensor.
3) Turn ignition switch to ON.
4) Measure voltage between front oxygen (A/
F) sensor connector and engine ground.
Connector & terminal
(B18) No. 4 (+) — Engine ground (
−−−−
):
Does the measured value exceed the spec-
ified value?
10 V
Repair power sup-
ply 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 resistance of harness between ECM
connector and chassis ground.
Connector & terminal
(B136) No. 5 — Chassis ground:
(B136) No. 16 — Chassis ground:
Is the measured value less than the specified
value?
5
Ω
Repair harness
and connector.
NOTE:
In this case, repair
the following:
• Open circuit in
harness between
ECM and engine
ground terminal
• Poor contact in
ECM connector
Poor contact in
coupling connector
4
CHECK CURRENT DATA.
1) Start engine
2) Read data of front oxygen (A/F) sensor
heater current using Subaru Select Monitor
or OBD-II general scan tool.
Is the measured value less than the speci-
fied value?
NOTE:
•Subaru Select Monitor
For detailed operation procedure, refer to the
“READ CURRENT DATA FOR ENGINE”.
<Ref. to EN(H4SO)-32, Subaru Select Moni-
tor.>
•OBD-II scan tool
For detailed operation procedures, refer to the
OBD-II General Scan Tool Instruction Manual.
0.2 A
Repair poor con-
tact in connector.
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 voltage between ECM connector
and chassis ground.
Connector & terminal
(B136) No. 6 (+) — Chassis ground (
−−−−
):
(B136) No. 17 (+) — Chassis ground (
−−−−
):
Is the measured value less than the speci-
fied value?
1.0 V
EN(H4SO)-94
ENGINE (DIAGNOSTICS)
DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)
6
CHECK OUTPUT SIGNAL FROM ECM.
Measure voltage between ECM connector and
chassis ground.
Connector & terminal
(B136) No. 6 (+) — Chassis ground (
−−−−
):
(B136) No. 17 (+) — 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 poor con-
tact in ECM con-
nector.
7
CHECK FRONT OXYGEN (A/F) SENSOR.
1) Turn ignition switch to OFF.
2) Measure resistance between front oxygen
(A/F) sensor connector terminals.
Terminals
No. 3— No. 4:
Is the measured value less than the speci-
fied value?
10
Ω
Repair harness
and connector.
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 front oxy-
gen (A/F) sensor.
<Ref. to
FU(H4SO)-41,
Front Oxygen (A/
F) Sensor.>
Step
Value
Yes
No
EN(H4SO)-95
ENGINE (DIAGNOSTICS)
DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)
MEMO:
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