DTC P0420 Catalyst System Efficiency Below Threshold (Bank 1): Wiring Diagram

Fig 1: HO2S - Wiring Diagram

Courtesy of SUBARU OF AMERICA, INC.

DIAGNOSTIC PROCEDURE

Step	Check	Yes	No
1 CHECK EXHAUST SYSTEM.  Check for gas leaks or air suction caused by loose or dislocated nuts and bolts, and open hole at exhaust pipes. NOTE: Check the following positions. Between cylinder head and front exhaust pipe Between front exhaust pipe and front catalytic converter Between front catalytic converter and rear catalytic converter Loose or improperly attached front oxygen (A/F) sensor or rear oxygen sensor	Is there any fault in exhaust system?	Repair or replace the exhaust system. Ref. to GENERAL DESCRIPTION .	Go to step 2.
2 CHECK WAVEFORM DATA ON THE SUBARU SELECT MONITOR (WHILE DRIVING).  Drive at a constant speed of 80 -112 km/h (50 - 70 MPH). After 5 minutes have elapsed in the condition of step 1), use the Subaru Select Monitor while still driving to read the waveform data. At normal condition Courtesy of SUBARU OF AMERICA, INC.SUBARU OF AMERICA, INC. At abnormal condition (numerous inversion) Courtesy of SUBARU OF AMERICA, INC.SUBARU OF AMERICA, INC.	Is a normal waveform displayed?	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.	Go to step 3.
3 CHECK WAVEFORM DATA ON THE SUBARU SELECT MONITOR (WHILE IDLING).  Run the engine at idle. In the state of step 1), read the waveform data using the Subaru Select Monitor. At normal condition Courtesy of SUBARU OF AMERICA, INC.SUBARU OF AMERICA, INC.SUBARU OF AMERICA, INC. At abnormal condition 1 (numerous inversion Courtesy of SUBARU OF AMERICA, INC.SUBARU OF AMERICA, INC.SUBARU OF AMERICA, INC. At abnormal condition 2 (noise input) Courtesy of SUBARU OF AMERICA, INC.SUBARU OF AMERICA, INC.SUBARU OF AMERICA, INC.	Is a normal waveform displayed?	Go to step 4.	The waveform is displayed at abnormal condition 1: Go to step 4.  The waveform is displayed at abnormal condition 2: Go to step 5.
4 CHECK CATALYTIC CONVERTER.	Is the catalytic converter damaged?	Replace the catalytic converter. Ref. to FRONT CATALYTIC CONVERTER .	Go to step 5.
5 CHECK REAR OXYGEN SENSOR CONNECTOR AND COUPLING CONNECTOR.	Has water entered the connector?	Completely remove any water inside.	Go to step 6.
6 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 - (E61) No. 3:  (B135) No. 30 - (E61) No. 4:  (B135) No. 28 - (E25) No. 3:  (B135) No. 30 - (E25) No. 4:	Is the resistance less than 1 Ω?	Go to step 7.	Repair the harness and connector. NOTE: In this case, repair the following item: Open circuit in harness between ECM and rear oxygen sensor connector Poor contact of coupling connector
7 CHECK HARNESS BETWEEN ECM AND REAR OXYGEN SENSOR CONNECTOR.  Connect the connector to ECM. Turn the ignition switch to ON. Measure the voltage between rear oxygen sensor connector and chassis ground. Connector & terminal  (E61) No. 3 (+) - Chassis ground (-):  (E25) No. 3 (+) - Chassis ground (-):	Is the voltage 0.2 - 0.5 V?	Go to step 8.	Repair the harness and connector. NOTE: Repair the following locations. Open circuit in harness between ECM and rear oxygen sensor connector Poor contact of ECM connector Poor contact of coupling connector
8 CHECK REAR OXYGEN SENSOR SHIELD.  Turn the ignition switch to OFF. Expose the rear oxygen sensor connector body side harness sensor shield. Measure the resistance between sensor shield and chassis ground.	Is the resistance less than 1 Ω?	Replace the rear oxygen sensor. Ref. to REAR OXYGEN SENSOR .	Repair the open circuit of rear oxygen sensor harness.