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.

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 (diag)>DIAGNOSTIC TROUBLE CODE (DTC) .

Is DTC other than P0420 displayed? (Current malfunction)

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 (diag)>FREEZE FRAME DATA . Go to  2.

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 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  4.

1. Start the engine and warm up completely.
2. Read the waveform data of rear oxygen sensor during idling using Subaru Select Monitor.
3. Check the waveform by waggling the connector between ECM and rear oxygen sensor.
   • 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 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 .