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

Engine electrical system, 2.5 L model (without push button start). See appropriate SYSTEM WIRING DIAGRAM article.
Engine electrical system, 2.5 L model (with push button start). See appropriate SYSTEM WIRING DIAGRAM article.

Fig 1: Catalyst System Efficiency Below Threshold - Circuit Diagram (Bank 1)
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. Refer to GENERAL DESCRIPTION. .	Go to step 2 .
2 CHECK WAVEFORM DATA ON THE SUBARU SELECT MONITOR (WHILE DRIVING). Drive at a constant speed between 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 At abnormal condition (numerous inversion)	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 condition of step 1), use the Subaru Select Monitor to read the waveform data. At normal condition At abnormal condition 1 (numerous inversion) At abnormal condition 2 (noise input)	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. Refer 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. 21 - (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: Repair the following locations. Open circuit in harness between ECM and rear oxygen sensor connector Poor contact of coupling connector
7 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?	Go to step 8 .	Repair the open circuit of rear oxygen sensor harness.
8 CHECK ENGINE OIL AMOUNT AND EXHAUST GAS. Check the engine oil amount. Refer to INSPECTION . Check exhaust gas during idling.	Does the engine oil amount drop or white smoke emit from the muffler?	Check the engine, and repair the defective part. Refer to INSPECTION . After repairing the engine, replace the front catalytic converter. Refer to FRONT EXHAUST PIPE . After the catalytic converter is replaced, perform step 2 to check the normal waveform is displayed, and then exit the procedure.	Go to step 9 .
9 CHECK IGNITION SYSTEM. Check the spark plug. Refer to INSPECTION . Check the status of the ignition coil connector and the spark plug terminal.	Is there any fault in the ignition system?	After repairing the ignition system, replace the front catalytic converter. Refer to FRONT EXHAUST PIPE . After the catalytic converter is replaced, perform step 2 to check the normal waveform is displayed, and then exit the procedure.	Go to step 10 .
10 CHECK FUEL SYSTEM. Refer to and check the items in "Insufficient fuel supply to fuel injector (except for "a. Fuel pump does not operate.")" and "Leakage or blow out of fuel". Refer to INSPECTION . Check throttle body. Refer to INSPECTION . Check intake manifold. Refer to INSPECTION .	Is there any fault in the fuel system?	After repairing the fuel system, replace the front catalytic converter. Refer to FRONT EXHAUST PIPE . After the catalytic converter is replaced, perform step 2 to check the normal waveform is displayed, and then exit the procedure.	Go to step 11 .
11 CHECK DTC.	Is any other DTC displayed?	Check the appropriate DTC using the "List of Diagnostic Trouble Code (DTC)". Refer to LIST OF DIAGNOSTIC TROUBLE CODE (DTC) . After checking the DTC, replace the front catalytic converter. Refer to FRONT EXHAUST PIPE . After the catalytic converter is replaced, perform step 2 to check the normal waveform is displayed, and then exit the procedure.	Replace the rear oxygen sensor. Refer to REAR OXYGEN SENSOR .