Circuit Description
The ECM compares the waveform of the heated oxygen sensor (HO2S) located in front of the catalyst with the waveform of the heated oxygen sensor located behind the catalyst to determine whether or not catalyst performance has deteriorated.
Air-fuel ratio feedback compensation keeps the waveform of the heated oxygen sensor in front of the catalyst alternating from rich to lean.
If the catalyst is functioning normally, the waveform of the heated oxygen sensor behind the catalyst switches back and forth between rich and lean much more slowly than the waveform of the heated oxygen sensor in front of the catalyst.
But when both waveforms change at a similar rate, it indicates that catalyst performance has deteriorated.