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.

Code is set if after driving vehicle for 5 minutes at 20-50 MPH and engine is at normal operating temperature, oxygen sensor waveforms are the same. Code P0420 is for the left side. Code P0430 is for the right side. Possible causes are:

• Catalytic converter malfunction.
• Open or short circuit in oxygen sensor circuit.
• Oxygen sensor malfunction.