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

DTC detecting condition: 

Detected when two consecutive driving cycles with fault occur.

Trouble symptom: 

Wiring diagram: 

Engine Electrical System Refer to WIRING SYSTEM>ENGINE ELECTRICAL SYSTEM>WIRING DIAGRAM .

G11577050Courtesy of SUBARU OF AMERICA, INC.
  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?

    Yes:  Repair or replace the exhaust system. Refer to EXHAUST(W/O STI)>GENERAL DESCRIPTION .

    No:  Go to  2.

  2. CHECK WAVEFORM DATA ON THE SUBARU SELECT MONITOR (WHILE DRIVING). 
    1. Drive at a constant speed between 80 - 112 km/h (50 - 70 MPH).
    2. 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
        G05954558Courtesy of SUBARU OF AMERICA, INC.
      • At abnormal condition (numerous inversion)
        G05954559Courtesy of SUBARU OF AMERICA, INC.

    Is a normal waveform displayed?

    Yes:  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.

    No:  Go to  3.

  3. CHECK WAVEFORM DATA ON THE SUBARU SELECT MONITOR (WHILE IDLING). 
    1. Run the engine at idle.
    2. In the condition of step 1), use the Subaru Select Monitor to read the waveform data.
      • At normal condition
        G13650613Courtesy of SUBARU OF AMERICA, INC.
      • At abnormal condition 1 (numerous inversion)
        G13650614Courtesy of SUBARU OF AMERICA, INC.
      • At abnormal condition 2 (noise input)
        G13650615Courtesy of SUBARU OF AMERICA, INC.

    Is a normal waveform displayed?

    Yes:  Go to  4.

    No  :

    • The waveform is displayed at abnormal condition 1: Go to  4.
    • The waveform is displayed at abnormal condition 2: Go to  5.
  4. CHECK CATALYTIC CONVERTER. 

    Check the catalytic converter. Is the catalytic converter damaged?

    Yes:  Replace the catalytic converter. Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (W/O STI)>FRONT CATALYTIC CONVERTER . Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (W/O STI)>REAR CATALYTIC CONVERTER .

    No:  Go to  5.

  5. CHECK REAR OXYGEN SENSOR CONNECTOR. 

    Check the rear oxygen sensor connector. Has water entered the connector?

    Yes:  Completely remove any water inside.

    No:  Go to  6.

  6. CHECK HARNESS BETWEEN ECM AND REAR OXYGEN SENSOR CONNECTOR. 
    1. Turn the ignition switch to OFF.
    2. Disconnect the connector from ECM.
    3. Disconnect the connector from rear oxygen sensor.
    4. Measure the resistance of harness between ECM connector and rear oxygen sensor connector.

      Connector & terminal 

      (E158) No. 42 - (E25) No. 3:

      (E158) No. 16 - (E25) No. 4:

    Is the resistance less than 1 Ω?

    Yes:  Go to  7.

    No:  Repair the open circuit of harness between ECM connector and rear oxygen sensor connector.

  7. CHECK REAR OXYGEN SENSOR SHIELD. 
    1. Turn the ignition switch to OFF.
    2. Expose the rear oxygen sensor connector body side harness sensor shield.
    3. Measure the resistance between sensor shield and engine ground.

    Is the resistance less than 1 Ω?

    Yes:  Go to  8.

    No:  Repair the open circuit of rear oxygen sensor harness.

  8. CHECK ENGINE OIL AMOUNT AND EXHAUST GAS. 
    1. Check the engine oil amount. Refer to LUBRICATION (W/O STI)>ENGINE OIL>INSPECTION .
    2. Check exhaust gas during idling.

    Does the engine oil amount drop or white smoke emit from the muffler?

    Yes:  Check the engine, and repair the defective part. Refer to MECHANICAL(W/O STI)>SYMPTOMS AND CAUSES>INSPECTION .

    After repairing the engine, replace the catalytic converter. Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (W/O STI)>FRONT CATALYTIC CONVERTER . Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (W/O STI)>REAR CATALYTIC CONVERTER .

    After the catalytic converter is replaced, perform step  2 to check the normal waveform is displayed, and then exit the procedure.

    No:  Go to  9.

  9. CHECK IGNITION SYSTEM. 
    1. Check the spark plug. Refer to IGNITION(W/O STI)>SPARK PLUG>INSPECTION .
    2. Check the status of the ignition coil connector and the spark plug terminal.

    Is there any fault in the ignition system?

    Yes:  After repairing the ignition system, replace the catalytic converter. Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (W/O STI)>FRONT CATALYTIC CONVERTER . Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (W/O STI)>REAR CATALYTIC CONVERTER .

    After the catalytic converter is replaced, perform step  2 to check the normal waveform is displayed, and then exit the procedure.

    No:  Go to  10.

  10. CHECK FUEL SYSTEM. 
    1. 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 FUEL INJECTION (FUEL SYSTEMS) (W/O STI)>FUEL SYSTEM TROUBLE IN GENERAL>INSPECTION .
    2. Check throttle body. Refer to FUEL INJECTION (FUEL SYSTEMS) (W/O STI)>THROTTLE BODY>INSPECTION .
    3. Check intake manifold. Refer to FUEL INJECTION (FUEL SYSTEMS) (W/O STI)>INTAKE MANIFOLD>INSPECTION .

    Is there any fault in the fuel system?

    Yes:  After repairing the fuel system, replace the catalytic converter. Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (W/O STI)>FRONT CATALYTIC CONVERTER . Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (W/O STI)>REAR CATALYTIC CONVERTER .

    After the catalytic converter is replaced, perform step  2 to check the normal waveform is displayed, and then exit the procedure.

    No:  Go to  11.

  11. CHECK DTC. 

    Read the engine DTC using the Subaru Select Monitor or general scan tool. Refer to ENGINE (DIAGNOSTICS) (W/O STI)>READ DIAGNOSTIC TROUBLE CODE (DTC) .

    Is other than DTC P0420 displayed? (Current malfunction)

    Yes:  Check the appropriate DTC using the "List of Diagnostic Trouble Code (DTC)". Refer to ENGINE (DIAGNOSTICS) (W/O STI)>LIST OF DIAGNOSTIC TROUBLE CODE (DTC) .

    After checking the DTC, replace the catalytic converter. Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (W/O STI)>FRONT CATALYTIC CONVERTER . Refer to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (W/O STI)>REAR CATALYTIC CONVERTER .

    After the catalytic converter is replaced, perform step  2 to check the normal waveform is displayed, and then exit the procedure.

    No:  Replace the rear oxygen sensor. Refer to FUEL INJECTION (FUEL SYSTEMS) (W/O STI)>REAR OXYGEN SENSOR .

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