Circuit Description
WARNING: This page is about a different car, the 2003 Toyota 4Runner. However, it is still accessible from the selected car via links, so may be relevant.
The ECM observes the waveform of the oxygen sensor located behind the catalyst to determine whether the catalyst performance has deteriorated. If the catalyst is functioning normally, the waveform of the oxygen sensor located behind the catalyst switches back and forth between rich and lean much more slowly. When the waveform of the oxygen sensor located behind the catalyst alternates frequently between rich and lean, it indicates that catalyst performance has deteriorated. See Figure.