ENGINE (DIAGNOSTICS)(H4SO) > Diagnostic Procedure with Diagnostic Trouble Code (DTC) > DTC P0031 HO2S HEATER CONTROL CIRCUIT LOW (BANK 1 SENSOR 1)

ENGINE (DIAGNOSTICS)(H4SO) > Diagnostic Procedure with Diagnostic Trouble Code (DTC)

DTC P0031 HO2S HEATER CONTROL CIRCUIT LOW (BANK 1 SENSOR 1)

DTC DETECTING CONDITION:

Immediately at fault recognition

CAUTION:

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

WIRING DIAGRAM:

STEP	CHECK	YES	NO
1.CHECK ANY OTHER DTC ON DISPLAY.	Are DTCs P0031 and P0037 displayed together on the Subaru Select Monitor?
2.CHECK POWER SUPPLY TO FRONT OXYGEN (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 (E23) No. 6 (+) — Engine ground (−):	Is the voltage 10 V or more?		Repair the power supply line. NOTE: In this case, repair the following item: • Open circuit of harness between main relay and front oxygen (A/F) sensor connector • Poor contact in front oxygen (A/F) sensor connector • Poor contact of main relay connector
3.CHECK GROUND CIRCUIT FOR ECM. Measure the resistance of harness between ECM connector and chassis ground. Connector & terminal (B134) No. 5 — Chassis ground: (B137) No. 7 — Chassis ground: (B137) No. 1 — Chassis ground: (B137) No. 2 — Chassis ground: (B137) No. 3 — Chassis ground:	Is the resistance less than 5 Ω?		Repair the harness and connector. NOTE: In this case, repair the following item: • Open circuit of harness between ECM and engine ground terminal • Poor contact in ECM connector • Poor contact of coupling connector
4.CHECK CURRENT DATA. 1) Start the engine. 2) Read the data of front oxygen (A/F) sensor heater current using Subaru Select Monitor. NOTE: For detailed operation procedure, refer to “READ CURRENT DATA FOR ENGINE”.	Is the current 0.2 A or more?	Repair the poor contact of connector. NOTE: In this case, repair the following item: • Poor contact in front oxygen (A/F) sensor connector • Poor contact in ECM connector
5.CHECK INPUT SIGNAL OF ECM. 1) Start and idle the engine. 2) Measure the voltage between ECM connector and chassis ground. Connector & terminal (B136) No. 2 (+) — Chassis ground (−): (B136) No. 3 (+) — Chassis ground (−):	Is the voltage less than 1 V?
6.CHECK OUTPUT SIGNAL OF ECM. Measure the voltage between ECM connector and chassis ground. Connector & terminal (B136) No. 2 (+) — Chassis ground (−): (B136) No. 3 (+) — Chassis ground (−):	Does the voltage change by shaking the harness and connector of ECM while monitoring the value on voltage meter?	Repair the poor contact of ECM connector.
7.CHECK FRONT OXYGEN (A/F) SENSOR. 1) Turn the ignition switch to OFF. 2) Measure the resistance between front oxygen (A/F) sensor connector terminals. Terminals No. 4 — No. 6:	Is the resistance less than 10 Ω?	Repair the harness and connector. NOTE: In this case, repair the following item: • Open or ground short circuit of harness between front oxygen (A/F) sensor and ECM connector • Poor contact in front oxygen (A/F) sensor connector • Poor contact in ECM connector	Replace the front oxygen (A/F) sensor.