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P0420-CATALYST 1/1 EFFICIENCY (CONTINUED) For the Engine circuit diagram (Refer to 9 - ENGINE - SCHEMATICS AND DIAGRAMS). For a complete wiring diagram Refer to Section 8W. Theory of Operation The upstream O2 sensor is used to detect the amount of oxygen in the exhaust gas before the gas enters the • When Monitored: • Set Condition: Possible Causes EXHAUST LEAK ENGINE MECHANICAL CONDITION AGING O2 SENSOR CATALYTIC CONVERTER Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE - Diagnostic Test 1. ACTIVE DTC NOTE: A new rear O2 Sensor along with an aging front O2 Sensor may cause the DTC to set. Review the NOTE: If an O2 Sensor DTC set along with the Catalytic Converter Efficiency DTC diagnose the O2 Sensor NOTE: Check for contaminants that may have damaged the O2 Sensor and Catalytic Converter: contami- With a scan tool, read DTCs. Is the DTC active at this time? Yes >> Go To 2 No >> Refer to the INTERMITTENT CONDITION Diagnostic Procedure. Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE) HB ENGINE ELECTRICAL DIAGNOSTICS 9 - 463 |