Diagnostic Aids

Check for any of the following conditions:

• A contaminated or malfunctioning HO2S can cause inaccurate sensor readings. For checks of the HO2S refer to Diagnostic Aids in DTC P2195 .
• Observe the HO2S 1 voltage and fuel trim readings with a scan tool. If the HO2S 1 voltage is fixed low, while the fuel trim is constantly high, check the mass air flow (MAF) sensor signal circuit for resistance. High circuit resistance can cause the sensor to read a lower volume of air than actual, resulting in a lean fueling condition. This lean fueling condition can cause the HO2S 2 voltage to be fixed low, causing the DTC P0420 diagnostic to fail.
• A damaged HO2S wiring harness-Inspect the HO2S wiring harnesses for damage and repair as necessary.
• If a DTC P0420 is intermittent, driving the vehicle under the conditions outlined in the Inspection/Maintenance (I/M) section can verify whether the fault is present. Refer to Inspection/Maintenance (I/M) Catalyst System Set Procedure .

A catalytic converter failure may be caused by any of the following conditions:

• An engine misfire
• An inadequate secondary ignition voltage
• Any excessive engine oil or engine coolant consumption
• A rich engine fueling

An intermittent malfunction may be caused by faults in the HO2S electrical circuits. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions .

The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.