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 compares the waveform of the heated oxygen sensor located before the catalyst with the waveform of the heated oxygen sensor located after the catalyst to determine whether or not the catalyst performance has deteriorated. The data input to ECM keeps the HO2S (Sensor 1) repeatedly changing back and forth from rich to lean. If the catalyst is functioning normally, the HO2S (Sensor 2) after the catalyst switches back and forth between rich and lean much more slowly than the HO2S (Bank 1) before the catalyst. But when both HO2S (Sensor 1 & 2) change at a similar rate, it indicates that the catalyst performance has deteriorated. DTC is set when HO2S (Sensor 1 & 2) have nearly the same waveform pattern after engine and catalytic converter are at normal operating temperature and vehicle is driven. Possible causes are:

Exhaust system leak.
Defective HO2S.
Defective 3-way catalytic converter (in exhaust manifold).