Wiring Diagram:

WARNING: This page is about a different car, the 2012 Subaru Forester. However, it is still accessible from the selected car via links, so may be relevant.

Fig 1: O2 Sensor Slow Response - Rich To Lean (Bank 1 Sensor 2)

Courtesy of SUBARU OF AMERICA, INC.

DIAGNOSIS PROCEDURE

Step	Check	Yes	No
1 CHECK HARNESS BETWEEN ECM AND REAR OXYGEN SENSOR CONNECTOR.. Turn the ignition switch to OFF. Disconnect the connector from ECM and rear oxygen sensor Measure the resistance of harness between ECM and rear oxygen sensor connector. Connector & terminal (B136) No. 20 - (E22) No. 3	Is the resistance less than 1 Ω?	Go to step 2	Repair the harness and connector. NOTE: Open circuit in harness between ECM and rear oxygen sensor connector Poor contact of coupling connector
2 CHECK HARNESS BETWEEN ECM AND REAR OXYGEN SENSOR CONNECTOR. Measure the resistance between rear oxygen sensor connector and chassis ground. Connector & terminal (E22) No. 3 - Chassis ground	Is the resistance 1 MΩ or more?	Go to step 3 .	Repair the ground short circuit of harness between ECM and rear oxygen sensor connector.
3 CHECK REAR OXYGEN SENSOR. Measure the resistance between rear oxygen sensor terminals. Terminals No. 3 - No. 4	Is the resistance less than 1 Ω?	Replace the rear oxygen sensor. Refer to REAR OXYGEN SENSOR	Even if DTC is detected, the circuit has returned to a normal condition at this time. Reproduce the failure, and then perform the diagnosis again. NOTE: In this case, temporary poor contact of connector, temporary open or short circuit of harness may be the cause.