Circuit Description

The ECM compares waveform of oxygen sensor located in front of catalytic converter with the waveform of oxygen sensor located behind converter to determine if converter performance has deteriorated. If converter is functioning properly, waveform of oxygen sensor after converter will switch back and forth between rich and lean much more slowly than waveform of oxygen sensor in front of converter.

DTC P0420 is set if after vehicle has warmed up waveforms of heated oxygen sensors (bank 1 and bank 2) have same amplitude, while vehicle and engine speeds are held at a steady rate. Possible causes are:

• Catalytic Converter
• Open Or Short Circuit In Oxygen Sensor Circuit
• Oxygen Sensor