ENGINE (DIAGNOSTICS)(H4DO) > Diagnostic Procedure with Diagnostic Trouble Code (DTC)

DTC P0138 O2 SENSOR CIRCUIT HIGH VOLTAGE (BANK 1 SENSOR 2)

DTC DETECTING CONDITION:

Two consecutive driving cycles with fault

CAUTION:

After repairing or replacing the defective part, perform the Clear Memory Mode and Inspection Mode .

WIRING DIAGRAM:

STEPCHECKYESNO

1.CHECK ANY OTHER DTC ON DISPLAY.

Is any other DTC displayed?

Check DTC using “List of Diagnostic Trouble Code (DTC)”.

2.CHECK REAR OXYGEN SENSOR DATA.

1) Warm-up the engine until engine coolant temperature is above 70°C (158°F), and rapidly reduce the engine speed from 3,000 rpm.

2) Read the data of rear oxygen sensor signal using Subaru Select Monitor or general scan tool.

NOTE:

• For MT model, depress the clutch pedal.

• Subaru Select Monitor

For detailed operation procedure, refer to “READ CURRENT DATA FOR ENGINE”.

• General scan tool

For detailed operation procedures, refer to the “General Scan Tool Instruction Manual”.

Is the voltage 250 mV or less?

3.CHECK REAR OXYGEN SENSOR CONNECTOR AND COUPLING CONNECTOR.

Does water enter the connector?

Dry the water thoroughly.

4.CHECK HARNESS BETWEEN ECM AND REAR OXYGEN SENSOR CONNECTOR.

1) Turn the ignition switch to OFF.

2) Disconnect the connectors from ECM and rear oxygen sensor.

3) Measure the resistance of harness between ECM and rear oxygen sensor connector.

Connector & terminal

(B136) No. 21 — (E22) No. 4: (B137) No. 31 — (E22) No. 3:

Is the resistance more than 3 Ω?

Repair the open circuit of harness between ECM and rear oxygen sensor connector.

5.CHECK HARNESS BETWEEN REAR OXYGEN SENSOR AND ECM CONNECTOR.

1) Turn the ignition switch to OFF.

2) Disconnect the connector from the rear oxygen sensor.

3) Turn the ignition switch to ON.

4) Measure the voltage between rear oxygen sensor harness connector and chassis ground.

Connector & terminal

(E22) No. 4 (+) — Chassis ground (−):

Is the voltage 0.2 — 0.5 V?

Replace the rear oxygen sensor.

Repair the harness and connector.

NOTE:

In this case, repair the following item:

• Open circuit of harness between the rear oxygen sensor and ECM connector

• Poor contact of the rear oxygen sensor connector

• Poor contact in ECM connector

6.CHECK EXHAUST SYSTEM.

Check exhaust system parts.

NOTE:

Check the following items.

• Loose part and incomplete installation of exhaust system

• Damage (crack, hole etc.) of parts

• Loose part and improper installation between front oxygen (A/F) sensor and rear oxygen sensor

Is there any fault in exhaust system?

Repair or replace faulty parts.

Replace the rear oxygen sensor.