Subaru Impreza 2003 Service Manual page 854

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)
ENGINE (DIAGNOSTICS)
Step
1 CHECK FOR OTHER CAUSES AFFECTING
EXHAUST GAS.
NOTE:
•Check for use of improper fuel.
•Check if engine oil or coolant level is
extremely low.
2 CHECK EXHAUST SYSTEM.
3 CHECK INPUT VOLTAGE FOR OXYGEN
SENSOR.
1)Disconnect the connector from oxygen sen-
sor connector.
2)Measure the voltage between main relay and
oxygen sensor.
Connector & terminal
(B18) No. 2 (+) — Chassis ground ( − − − − ):
4 CHECK HARNESS CONNECTOR BETWEEN
OXYGEN SENSOR AND ENGINE GROUND
CABLE.
Measure the resistance between oxygen sen-
sor and chassis ground.
Connector & terminal
(B18) No. 1 — Chassis ground:
5 CHECK OXYGEN SENSOR.
Measure the resistance between oxygen sen-
sor terminals.
Connector & terminal
No. 1 — No. 2:
6 CHECK HARNESS BETWEEN ECM AND OX-
YGEN SENSOR.
1)Disconnect the connector from ECM.
2)Measure the resistance between ECM and
chassis ground.
Connector & terminal
(B136) No. 7 — Chassis ground:
7 CHECK HARNESS BETWEEN ECM AND OX-
YGEN SENSOR.
1)Turn the ignition switch to ON.
2)Measure the voltage between ECM and
chassis ground.
Connector & terminal
(B136) No. 7 (+) — Chassis ground (–):
Check
Is CO % more than 2% after
the engine warm up?
Is there a fault in the exhaust
system?
Is the voltage more than 10 V? Go to step 4.
Is the resistance less than 1
Ω?
Is the resistance less than 30
Ω?
Is the resistance more than 1
MΩ?
Is the voltage more than 0.2 V? Repair the battery
EN(H4SOw/oOBD)-78
Yes
Check the fuel Go to step 2.
system.
Repair the exhaust Go to step 3.
system.
NOTE:
• Loose installa-
tion of front portion
of exhaust pipe
onto cylinder
heads
• Loose connec-
tion between front
exhaust pipe and
front catalytic con-
verter
• Damage of
exhaust pipe
resulting in hole
Repair the open
circuit between
main relay and
oxygen sensor.
Go to step 5. Repair the open
circuit between
oxygen sensor
and chassis
ground.
Repair poor con- Go to step 6.
tact.
Go to step 7. Repair the ground
short circuit
between ECM and
chassis ground.
Go to step 8.
short circuit
between ECM and
oxygen sensor.
No