ENGINE (DIAGNOSTICS)(H4DOTC) > Diagnostic Procedure with Diagnostic Trouble Code (DTC)

DTC P0420 CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD (BANK 1)

DTC DETECTING CONDITION:

Detected when two consecutive driving cycles with fault occur.

TROUBLE SYMPTOM:

• Engine stalls.

• Idle mixture is out of specifications.

CAUTION:

After servicing or replacing faulty parts, perform Clear Memory Mode and Inspection Mode

WIRING DIAGRAM:

STEPCHECKYESNO

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.

2.CHECK WAVEFORM DATA ON THE SUBARU SELECT MONITOR (WHILE DRIVING).

1) Drive at a constant speed of 80 — 112 km/h (50 — 70 MPH).

2) After 5 minutes have elapsed in the state of step 1), read the wave form data using the Subaru Select Monitor while driving.

Is a normal waveform displayed?

Even if the malfunction indicator light illuminates, the circuit has returned to a normal condition at this time. Reproduce the fault condition, and reperform the check.

NOTE:

In this case, there may be a temporary connector contact failure.

3.CHECK WAVEFORM DATA ON THE SUBARU SELECT MONITOR (WHILE IDLING).

1) Idle the engine.

2) In the state of step 1), read the waveform data using the Subaru Select Monitor.

Is a normal waveform displayed?

4.CHECK CATALYTIC CONVERTER.

Is the catalytic converter damaged?

Replace the catalytic converter.

5.CHECK REAR OXYGEN SENSOR CONNECTOR AND COUPLING CONNECTOR.

Has water entered the connector?

Completely remove any water inside.

6.CHECK HARNESS BETWEEN ECM AND REAR OXYGEN SENSOR CONNECTOR.

1) Turn the ignition switch to OFF.

2) Disconnect the connector from ECM and rear oxygen sensor.

3) Measure the resistance of harness between ECM and rear oxygen sensor connector.

Connector & terminal

(B135) No. 4 — (T6) No. 3:

(B135) No. 30 — (T6) No. 4:

Is the resistance less than 1 Ω?

Repair the harness and connector.

NOTE:

In this case, repair the following items.

• Open circuit in harness between ECM and rear oxygen sensor connector

• Poor contact of coupling connector

7.CHECK HARNESS BETWEEN ECM AND REAR OXYGEN SENSOR CONNECTOR.

1) Turn the ignition switch to ON.

2) Measure the voltage between rear oxygen sensor connector and chassis ground.

Connector & terminal

(T6) No. 3 (+) — Chassis ground (−):

Is the voltage 0.2 — 0.5 V?

Repair the harness and connector.

NOTE:

Repair the following positions.

• Open circuit of harness between ECM and rear oxygen sensor

• Poor contact in ECM and rear oxygen sensor

• Poor contact in ECM connector

8.CHECK REAR OXYGEN SENSOR SHIELD.

1) Turn the ignition switch to OFF.

2) Expose the body side harness sensor shield of the rear oxygen sensor connector.

3) Measure the resistance between sensor shield and chassis ground.

Is the resistance less than 1 Ω?

Replace the rear oxygen sensor.

Repair the open circuit of rear oxygen sensor harness.