1.CHECK ANY OTHER DTC ON DISPLAY.
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Is there any other DTC displayed?
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Check the appropriate DTC using the “List of Diagnostic Trouble Code (DTC)”. 
NOTE:
In this case, it is not necessary to inspect DTC P0420.
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2.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
• Looseness and improper fitting of front oxygen (A/F) sensor or rear oxygen sensor
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Is there any fault in exhaust system?
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Repair or replace the exhaust system.
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3.CHECK WAVEFORM DATA ON SUBARU SELECT MONITOR (WHILE DRIVING).
1) Drive the vehicle at a constant speed of 80 — 112 km/h (50 — 70 MPH).
2) Keep the condition of step 1) for 5 minutes, then read the waveform data in a driving condition using Subaru Select Monitor.
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Is normal waveform pattern displayed?
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Contact the SUBARU distributor service.
NOTE:
The probable cause is considered as the deterioration of multiple parts.
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4.CHECK WAVEFORM DATA ON SUBARU SELECT MONITOR (WHILE IDLING).
2) Under the condition of step 1), read the waveform data using Subaru Select Monitor.
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Is normal waveform pattern displayed?
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5.CHECK REAR OXYGEN SENSOR VOLTAGE.
1) Warm-up the engine until engine coolant temperature is above 70°C (158°F), and keep the engine speed at 3,000 rpm. (Max. 2 min.)
2) Read the voltage of rear oxygen (A/F) sensor signal using Subaru Select Monitor.
NOTE:
• For MT model, depress the clutch pedal.
• Subaru Select Monitor
For detailed operation procedure, refer to “READ CURRENT DATA FOR ENGINE”.
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Is the voltage 490 mV or more?
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6.CHECK REAR OXYGEN SENSOR CONNECTOR AND COUPLING CONNECTOR.
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Does water enter the connector?
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Remove water thoroughly from the connector.
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7.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 — (E22) No. 3:
(B134) No. 29 — (E22) No. 4:
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Is the resistance 3 Ω or more?
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Repair the open circuit of harness between ECM and rear oxygen sensor connector.
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8.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
(E22) No. 3 (+) — Chassis ground (−):
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Is the voltage 0.2 — 0.5 V?
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Repair the harness and connector.
NOTE:
Repair the following points.
• Open circuit of harness between rear oxygen sensor and ECM connector
• Poor contact of the rear oxygen sensor and ECM connector
• Poor contact in ECM connector
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9.CHECK REAR OXYGEN SENSOR VOLTAGE.
1) Warm-up the engine until engine coolant temperature is above 70°C (158°F), and rapidly reduce the engine speed from 3,000 rpm.
2) Read the voltage of rear oxygen (A/F) sensor signal using Subaru Select Monitor.
NOTE:
• For MT model, depress the clutch pedal.
• Subaru Select Monitor
For detailed operation procedure, refer to “READ CURRENT DATA FOR ENGINE”.
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Is the voltage 250 mV or less?
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Contact the SUBARU distributor service.
NOTE:
The probable cause is considered as the deterioration of multiple parts.
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10.CHECK CATALYTIC CONVERTER.
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Is the catalytic converter damaged?
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Replace the catalytic converter.
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Contact the SUBARU distributor service.
NOTE:
The probable cause is considered as the deterioration of multiple parts.
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11.CHECK REAR OXYGEN SENSOR SHIELD.
1) Turn the ignition switch to OFF.
2) Bare the harness sensor shield on the body side of rear oxygen sensor connector.
3) Measure the resistance between sensor shield and chassis ground.
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Is the resistance less than 1 Ω?
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Replace the rear oxygen sensor.
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Repair the open circuit of rear oxygen sensor harness.
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