Subaru Impreza 3 / Impreza WRX / Impreza WRX STI. Manual — part 286

EN(H4DOTC)(diag)-368

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

ENGINE (DIAGNOSTICS)

CHECK ENGINE COOLANT TEMPERATURE
SENSOR.
1) Start the engine and warm up completely.
2) Read the value of «Coolant Temp.» using
the Subaru Select Monitor or a general scan
tool.
NOTE:

• Subaru Select Monitor
For detailed operation procedures, refer to
“READ CURRENT DATA FOR ENGINE”. <Ref.
to EN(H4DOTC)(diag)-40, Subaru Select Moni-
tor.>
• General scan tool
For detailed operation procedures, refer to the
general scan tool operation manual.

Is the value of «Coolant
Temp.» 75°C (167°F) or more?

Go to step

13.

Replace the
engine coolant
temperature sen-
sor. <Ref. to
FU(STI)-34,
Engine Coolant
Temperature Sen-
sor.> <Ref. to
FU(w/o STI)-34,
Engine Coolant
Temperature Sen-
sor.>

CHECK MASS AIR FLOW AND INTAKE AIR
TEMPERATURE SENSOR.
1) Start the engine and warm up engine until
coolant temperature is higher than 75°C
(167°F).
2) Place the shift lever in neutral position.
3) Turn the A/C switch to OFF.
4) Turn all the accessory switches to OFF.
5) Read the value of «Mass Air Flow» using
the Subaru Select Monitor or a general scan
tool.
NOTE:

Is the value of «Mass Air Flow»
2.0 — 5.0 g/s (0.26 — 0.66 lb/m)?

Go to step

14.

Replace the mass
air flow and intake
air temperature
sensor. <Ref. to
FU(STI)-42, Mass
Air Flow and Intake
Air Temperature
Sensor.> <Ref. to
FU(w/o STI)-42,
Mass Air Flow and
Intake Air Temper-
ature Sensor.>

CHECK MASS AIR FLOW AND INTAKE AIR
TEMPERATURE SENSOR.
1) Start the engine and warm up engine until
coolant temperature is higher than 75°C
(167°F).
2) Place the shift lever in neutral position.
3) Turn the A/C switch to OFF.
4) Turn all the accessory switches to OFF.
5) Open the front hood.
6) Measure the ambient temperature.
7) Read the value of «Intake Air Temp.» using
the Subaru Select Monitor or a general scan
tool.
NOTE:

Subtract ambient temperature
from «Intake Air Temp.». Is the
obtained value –10 — 50°C (–18
— 90°F)?

Go to step

15.

Check the mass air
flow and intake air
temperature sen-
sor. <Ref. to
FU(STI)-42, Mass
Air Flow and Intake
Air Temperature
Sensor.> <Ref. to
FU(w/o STI)-42,
Mass Air Flow and
Intake Air Temper-
ature Sensor.>

Step

Check

Yes

EN(H4DOTC)(diag)-369

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

ENGINE (DIAGNOSTICS)

CHECK REAR OXYGEN SENSOR DATA.
1) Warm up the engine until engine coolant
temperature is higher than 75°C (167°F), and
keep the engine speed at 3,000 rpm. (2 minutes
maximum)
2) Read the value of «Rear O2 Sensor» using
the Subaru Select Monitor or a general scan
tool.
NOTE:

• Depress the clutch pedal.
• Subaru Select Monitor
For detailed operation procedures, refer to
“READ CURRENT DATA FOR ENGINE”. <Ref.
to EN(H4DOTC)(diag)-40, Subaru Select Moni-
tor.>
• General scan tool
For detailed operation procedures, refer to the
general scan tool operation manual.

Is the value of «Rear O2 Sen-
sor» 0.490 V or more?

CHECK REAR OXYGEN SENSOR DATA.
1) Warm up the engine until engine coolant
temperature is higher than 75°C (167°F), and
rapidly reduce the engine speed from 3,000
rpm.
2) Read the value of «Rear O2 Sensor» using
the Subaru Select Monitor or a general scan
tool.
NOTE:

Is the value of «Rear O2 Sen-
sor» 0.250 V or less?

CHECK REAR OXYGEN SENSOR CONNEC-
TOR AND COUPLING CONNECTOR.

Has water entered the connec-
tor?

Completely
remove any water
inside.

Go to step

19.

CHECK FRONT OXYGEN (A/F) SENSOR US-
ING REAR OXYGEN SENSOR SIGNAL.
1) Warm up the engine until engine coolant
temperature is higher than 75°C (167°F), then
keep the engine idling for 5 minutes or more.
2) Read the value of «Rear O2 Sensor» using
the Subaru Select Monitor or a general scan
tool.
NOTE:

Is the value in «Rear O2 Sen-
sor» kept at 0.8 V or more for 5
minutes or more?

Replace the front
oxygen (A/F) sen-
sor. <Ref. to
FU(STI)-54, Front
Oxygen (A/F) Sen-
sor.> <Ref. to
FU(w/o STI)-53,
Front Oxygen (A/F)
Sensor.>

Go to step

19.

Step

Check

Yes

EN(H4DOTC)(diag)-370

Diagnostic Procedure with Diagnostic Trouble Code (DTC)

ENGINE (DIAGNOSTICS)

CHECK HARNESS BETWEEN ECM AND
REAR OXYGEN SENSOR CONNECTOR.
1) Turn the ignition switch to OFF.
2) Disconnect the connector from ECM and
rear oxygen sensor.
3) Measure the resistance of harness between
ECM connector and rear oxygen sensor con-
nector.

Connector & terminal

(B136) No. 20 — (T6) No. 3:
(B135) No. 30 — (T6) No. 4:

Is the resistance less than 1 Ω? Go to step

20.

Repair the harness
and connector.
NOTE:

In this case, repair
the following item:
• Open circuit in
harness between
ECM

connector

and rear oxygen
sensor connector
• Poor contact of
coupling connector

CHECK HARNESS BETWEEN ECM AND
REAR OXYGEN SENSOR CONNECTOR.
1) Connect the connector to ECM.
2) Turn the ignition switch to ON.
3) Measure the voltage between rear oxygen
sensor connector and chassis ground.

Connector & terminal

(T6) No. 3 (+) — Chassis ground (–):

Is the voltage 0.2 — 0.5 V?

Replace the rear
oxygen sensor.
<Ref. to FU(STI)-
56, Rear Oxygen
Sensor.> <Ref. to
FU(w/o STI)-55,
Rear Oxygen Sen-
sor.>

Repair the harness
and connector.
NOTE:

In this case, repair
the following item:
• Open circuit in
harness between
ECM

connector

and rear oxygen
sensor connector
• Poor contact of
ECM connector
• Poor contact of
coupling connector

Step

Check

Yes

EN(H4DOTC)(diag)-371