Subaru Impreza 2003 Service Manual page 651

DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)
Step
2 CHECK FRONT OXYGEN (A/F) SENSOR DA-
TA.
1)Start the engine.
2)While observing the Subaru Select Monitor
or OBD-II general scan tool screen, warm-up
the engine until coolant temperature is above
70°C (160°F).
If the engine is already warmed-up, operate at
idle speed for at least 1 minute.
3)Read the data of front oxygen (A/F) sensor
signal during idling using Subaru Select Moni-
tor 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 general scan tool
For detailed operation procedures, refer to the
OBD-II General Scan Tool Instruction Manual.
3 CHECK FRONT OXYGEN (A/F) SENSOR DA-
TA.
Race the engine at speeds from idling to 5,000
rpm for a total of 5 cycles.
NOTE:
•Air fuel ratio is rich at normal condition or dur-
ing racing.
•To increase engine speed to 5,000 rpm,
slowly depress accelerator pedal, taking
approximately 5 seconds, and quickly release
accelerator pedal to decrease engine speed.
4 CHECK HARNESS BETWEEN ECM AND
FRONT OXYGEN (A/F) SENSOR.
1)Turn the ignition switch to OFF.
2)Disconnect the connector from ECM and
front oxygen (A/F) sensor connector.
3)Measure the resistance between ECM and
front oxygen (A/F) sensor.
Connector & terminals
(B136) No. 13 — (B18) No. 1:
(B136) No. 22 — (B18) No. 2:
5 CHECK HARNESS BETWEEN ECM AND
FRONT OXYGEN (A/F) SENSOR.
Measure the resistance between ECM and
chassis ground.
Connector & terminals
(B136) No. 13 — Chassis ground:
(B136) No. 22 — Chassis ground:
6 CHECK EXHAUST SYSTEM.
Check exhaust system parts.
NOTE:
Check the following items.
•Loose installation of portions
•Damage (crack, hole etc.) of parts
•Looseness of front oxygen (A/F) sensor
•Looseness and ill fitting of parts between front
oxygen (A/F) sensor and rear oxygen sensor
Check
Is the voltage 0.85 V — 1.15
V?
Is the voltage more than 1.1 V? Go to step 6.
Is the resistance less than 5
Ω?
Is the resistance more than 1
MΩ?
Is there a fault in exhaust sys-
tem?
EN(H4SO)-239
ENGINE (DIAGNOSTICS)
Yes
Go to step 3. Go to step 4.
Go to step 4.
Go to step 5. Repair open circuit
between ECM and
front oxygen (A/F)
sensor.
Go to step 6. Repair ground
short circuit
between ECM and
front oxygen (A/F)
sensor.
Repair or replace Replace the front
faulty parts. oxygen (A/F) sen-
sor. <Ref. to
FU(H4SO)-44,
Front Oxygen (A/
F) Sensor.>
No