DTC P0420: Catalyst System Efficiency Below Threshold Bank 1: Notes

DTC detecting condition: 

Detected when two consecutive driving cycles with fault occur.

Trouble symptom: 

• Engine stalls.
• Idle mixture is out of specifications.

Wiring diagram: 

Engine Electrical System Ref. to WIRING SYSTEM>Engine Electrical System>WIRING DIAGRAM

Courtesy of SUBARU OF AMERICA, INC.

1. CHECK DTC AND FREEZE FRAME DATA. 
   1. Read [Engine] DTC using the Subaru Select Monitor or general scan tool. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>Read Diagnostic Trouble Code (DTC)>OPERATION
   2. Read the freeze frame data. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>Subaru Select Monitor>OPERATION>DISPLAY OF ENGINE FREEZE FRAME DATA
   3. Store the displayed DTC and freeze frame data.

   Is other than DTC P0420 displayed? (Current malfunction)

   Yes:  Check the appropriate DTC using the "List of Diagnostic Trouble Code (DTC)". Ref. to ENGINE (DIAGNOSTICS) (H4DO)>LIST OF DIAGNOSTIC TROUBLE CODE (DTC)

   No:  Go to  2

2. CHECK ENGINE OIL CONSUMPTION. 

   Check the engine oil amount. Ref. to LUBRICATION(H4DO)>Engine Oil>INSPECTION

   Is white smoke emitted from muffler?

   Yes:  Check the engine, and repair the defective part. Ref. to MECHANICAL(H4DO)>ENGINE TROUBLE IN GENERAL After repairing the engine, replace all the catalytic converters. Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (H4DO)>FRONT CATALYTIC CONVERTER Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (H4DO)>REAR CATALYTIC CONVERTER

   No:  Go to  3

3. CHECK EXHAUST SYSTEM. 

   Check the following items for the exhaust system.

   • Looseness and disconnection of nut and bolt
   • Exhaust gas leakage
   • Air suction
   • Hole in exhaust pipe

   Is there any trouble?

   Yes:  Repair or replace the exhaust system. Ref. to EXHAUST(H4DO)>GENERAL DESCRIPTION

   No:  Go to  4

4. CHECK CATALYTIC CONVERTER. 
   1. Start the engine and warm up completely.
   2. Using the Subaru Select Monitor, read the waveform data of rear oxygen sensor during idling.
   3. Shake the connector between ECM and rear oxygen sensor and check the waveform.
      • At normal condition

        Courtesy of SUBARU OF AMERICA, INC.

      • At abnormal condition 2 (noise input)

        Courtesy of SUBARU OF AMERICA, INC.

   Is an abnormal waveform displayed?

   Yes:  Connect the connector after checking the following items, and check the waveform again while shaking the connector.

   • Check the connector connection condition of the section where waveform is abnormal, and repair or replace them if faulty.
   • When abnormality is not found, temporary poor contact of connector may be the cause.

   No:  Replace all the catalytic converters. Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (H4DO)>FRONT CATALYTIC CONVERTER Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (H4DO)>REAR CATALYTIC CONVERTER