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.

NOTE: After the faulty parts are repaired or replaced, perform the final check in Basic Diagnostic Procedure. Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>BASIC DIAGNOSTIC PROCEDURE>PROCEDURE .

CHECK DTC AND FREEZE FRAME DATA .
1. Turn the ignition switch to ON.
2. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to COMMON (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .
Is DTC other than P0420 displayed? (Current code)

Yes: Check the appropriate DTC using the "Diagnostic Trouble Code (DTC)". Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>DIAGNOSTIC TROUBLE CODE (DTC)>LIST .

No: Save the freeze frame data. Ref. to COMMON (DIAGNOSTICS)>FREEZE FRAME DATA . Go to 2.
CHECK ENGINE OIL CONSUMPTION .
Check the engine oil amount. Ref. to LUBRICATION (H4DOTC)>ENGINE OIL>INSPECTION .

Is white smoke emitted from the muffler?

Yes: Check the engine, and repair the defective part. Ref. to MECHANICAL (H4DOTC)>SYMPTOMS AND CAUSES .

After repairing the engine, replace all the catalytic converters. Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (H4DOTC)>FRONT CATALYTIC CONVERTER . Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (H4DOTC)>REAR CATALYTIC CONVERTER .

No: Go to 3.
CHECK EGR SYSTEM .
1. Turn the ignition switch to OFF.
2. Check for leakage in the pipes consisting of the EGR system.
  NOTE:
  Check the following items.
  - Looseness of each flange connection
  - Improper installation of throttle body
  - Improper installation of manifold absolute pressure and intake air temperature sensor
  - Foreign matter adhering to EGR control valve
Is the check result OK?

Yes: Go to 4.

No: Repair or replace faulty parts.
CHECK EXHAUST SYSTEM .
Check the following items regarding exhaust system.
- Loose or detached nut and bolt
- Exhaust gas leakage
- Air suction
- Hole on exhaust pipe
Is there any trouble?

Yes: Repair or replace the exhaust system. Ref. to EXHAUST (H4DOTC)>GENERAL DESCRIPTION .

No: Go to 5.
CHECK FRONT OXYGEN (A/F) SENSOR .
Visually check the front oxygen (A/F) sensor and the vicinity of front oxygen (A/F) sensor.

Is there any trace of oil on the front oxygen (A/F) sensor or in the vicinity of the front oxygen (A/F) sensor?

Yes: Go to 6.

No: Go to 7.

CHECK FRONT OXYGEN (A/F) SENSOR .

Start the engine and warm up completely.

Using the Subaru Select Monitor, decelerate by accelerator OFF from driving at a constant speed of 60 km/h (37.3 MPH), and read the waveform data of [A/F Sensor #1] and [Target Equivalence Ratio] while the vehicle is stopped.

At normal condition

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In malfunction

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Is an abnormal waveform displayed?

Yes: Replace the front oxygen (A/F) sensor. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DOTC)>FRONT OXYGEN (A/F) SENSOR .

No: Go to 7.

CHECK CATALYTIC CONVERTER .

Start the engine and warm up completely.
Read the waveform data of rear oxygen sensor during idling using an oscilloscope.

Check the waveform by waggling the connector between ECM and rear oxygen sensor.

At normal condition

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At abnormal condition 2 (noise input)

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Is an abnormal waveform displayed?

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

Check the contact point of the connector that caused disturbed waveform. If faulty, repair or replace it.
If any problem is not found, it is possible that a temporary poor contact occurs in the connector.

No: Replace all the catalytic converters. Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (H4DOTC)>FRONT CATALYTIC CONVERTER . Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (H4DOTC)>REAR CATALYTIC CONVERTER .