Wiring Diagram:

WARNING: This page is about a different variant/trim than selected.

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

G03830015

Courtesy of SUBARU OF AMERICA, INC.

DIAGNOSTIC PROCEDURE (DTC P0420)

Step		Check	Yes	No
1 CHECK ANY OTHER DTC ON DISPLAY.		Is any other DTC displayed?	Inspect the relevant DTC using "List of Diagnostic Trouble Code (DTC)". < Ref. to , LIST OF DIAGNOSTIC TROUBLE CODE (DTC) .> NOTE: In this case, it is not necessary to inspect DTC P0420.	Go to step 2.
2 CHECK HARNESS BETWEEN ECM AND REAR OXYGEN SENSOR CONNECTOR. Turn the ignition switch to OFF. Disconnect the connectors from ECM and rear oxygen sensor. Measure the resistance of harness between ECM and rear oxygen sensor connector. Connector & terminal (B137) No. 25 - (T6) No. 3: (B136) No. 35 - (T6) No. 4:		Is the resistance less than 1 ?	Go to step 3.	Repair open circuit in harness between ECM and rear oxygen sensor connector.
3 CHECK HARNESS BETWEEN REAR OXYGEN SENSOR AND ECM CONNECTOR. Measure the resistance between rear oxygen sensor harness connector and chassis ground. Connector & terminal (B137) No. 31 - Chassis ground:		Is the resistance less than 1 ?	Go to step 4.	Repair harness and connector. NOTE: In this case, repair the following: Open circuit in harness between rear oxygen sensor and ECM connector Poor contact in rear oxygen sensor connector Poor contact in ECM connector
4 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		Is there a fault in exhaust system?	Repair or replace the exhaust system. < Ref. to GENERAL DESCRIPTION . >	Go to step 5.
5 CHECK CATALYTIC CONVERTER.		Is there damage at rear face or front face of front catalyst?	Replace the catalytic converter. < Ref. to FRONT CATALYTIC CONVERTER . >	Contact Service Center.