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)

G03829045

Courtesy of SUBARU OF AMERICA, INC.

DTC P0420 DIAGNOSTIC PROCEDURE

Step	Check	Yes	No
1. CHECK FOR 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 FRONT OXYGEN SENSOR CONNECTOR. Turn the ignition switch to OFF. Disconnect the connectors from ECM and front oxygen sensor. Measure the resistance of harness between ECM and front oxygen sensor connector. Connector & terminal (B136) No. 24 - (B262) No. 3: (B136) No. 35 - (B262) No. 4: (B137) No. 25 - (B18) No. 3: (B136) 35 - (B18) No. 4:	Is the resistance less than 1 Ω?	Go to step 3.	Repair open circuit in harness between ECM and front oxygen sensor connector.
3. CHECK HARNESS BETWEEN FRONT OXYGEN SENSOR AND ECM CONNECTOR. Measure the resistance between front oxygen sensor harness connector and chassis ground. Connector & terminal (B137) No. 31 - Chassis ground:	Is the resistance less than 1 Ω?	Go to step 4.	Repair open circuit in harness between ECM and front oxygen sensor 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 any 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 SOA Service Center.