Subaru Impreza 2003 Service Manual page 494

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)
ENGINE (DIAGNOSTICS)
Step
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. 4 (+) — Engine ground ( − − − − ):
3 CHECK GROUND CIRCUIT OF ECM.
Measure the resistance of harness between
ECM connector and chassis ground.
Connector & terminal
(B136) No. 5 — Chassis ground:
(B136) No. 16 — Chassis ground:
4 CHECK CURRENT DATA.
1)Start the engine.
2)Read the data of front oxygen (A/F) sensor
heater current using Subaru Select Monitor or
OBD-II general scan tool.
NOTE:
•Subaru Select Monitor
For detailed operation procedure, refer to the
"READ CURRENT DATA FOR ENGINE".
<Ref. to EN(H4SO)-31, Subaru Select Moni-
tor.>
•OBD-II scan tool
For detailed operation procedures, refer to the
OBD-II General Scan Tool Instruction Manual.
5 CHECK INPUT SIGNAL FROM ECM.
1)Start and idle the engine.
2)Measure the voltage between ECM connec-
tor and chassis ground.
Connector & terminal
(B136) No. 6 (+) — Chassis ground ( − − − − ):
(B136) No. 17 (+) — Chassis ground ( − − − − ):
6 CHECK OUTPUT SIGNAL FROM ECM.
Measure the voltage between ECM connector
and chassis ground.
Connector & terminal
(B136) No. 6 (+) — Chassis ground ( − − − − ):
(B136) No. 17 (+) — Chassis ground ( − − − − ):
Check
Is the voltage more than 10 V? Go to step 3.
Is the resistance less than 5
Ω?
Is the current more than 0.2 A? Repair poor con-
Is the voltage less than 1 V?
Does the voltage change by
shaking the ECM harness and
connector?
EN(H4SO)-82
Yes
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
Go to step 4. 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
Go to step 5.
tact in connector.
NOTE:
In this case, repair
the following:
• Poor contact in
front oxygen (A/F)
sensor connector
• Poor contact in
ECM connector
Go to step 7. Go to step 6.
Repair poor con- Go to step 7.
tact in ECM con-
nector.
No