Subaru 2009 IMPREZA Service Manual page 1319

Diagnostic Procedure with Diagnostic Trouble Code (DTC)
ENGINE (DIAGNOSTICS)
Step
1 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
(E22) No. 4 (+) — Engine ground (–):
2 CHECK HARNESS BETWEEN ECM AND
FRONT OXYGEN (A/F) SENSOR CONNEC-
TOR.
1) Turn the ignition switch to OFF.
2) Disconnect the connectors from ECM.
3) Measure the resistance between ECM and
front oxygen (A/F) sensor connector.
Connector & terminal
(B136) No. 3 — (E22) No. 2:
(B136) No. 2 — (E22) No. 2:
3 CHECK GROUND CIRCUIT FOR ECM.
Measure the resistance of harness between
ECM and chassis ground.
Connector & terminal
(B134) No. 5 — Chassis ground:
(B137) No. 1 — Chassis ground:
(B137) No. 2 — Chassis ground:
(B137) No. 3 — Chassis ground:
(B137) No. 7 — Chassis ground:
4 CHECK FRONT OXYGEN (A/F) SENSOR.
Measure the resistance between front oxygen
(A/F) sensor terminals.
Terminals
No. 2 — No. 4:
Check
Is the voltage 10 V or more?
Is the resistance less than 1 ? Go to step 3.
Is the resistance less than 5 ? Go to step 4.
Is the resistance 2 — 3 ?
EN(H4DOTC)(diag)-98
Yes No
Go to step 2. Repair the power
supply line.
NOTE:
In this case, repair
the following item:
• Open circuit in
harness between
A/F, oxygen sen-
sor relay and front
oxygen (A/F) sen-
sor connector
• Poor contact of
A/F, oxygen sen-
sor relay connector
• Malfunction of A/
F, oxygen sensor
relay
• Poor contact of
coupling connector
Repair the harness
and connector.
NOTE:
In this case, repair
the following item:
• Open circuit in
harness between
ECM and front oxy-
gen (A/F) sensor
connector
• Poor contact of
coupling connector
Repair the harness
and connector.
NOTE:
In this case, repair
the following item:
• Open circuit of
harness between
ECM and engine
ground
• Poor contact of
coupling connector
Repair the poor Replace the front
contact of ECM oxygen (A/F) sen-
connector. sor. <Ref. to
FU(H4DOTC)-48,
Front Oxygen (A/F)
Sensor.>