Circuit Description

WARNING: This page is about a different car, the 2002 Toyota Prius. However, it is still accessible from the selected car via links, so may be relevant.

ECM compares waveform pattern of heated oxygen sensor No. 1 on exhaust pipe in front of catalytic converter with waveform pattern of heated oxygen sensor No. 2 on exhaust pipe behind catalytic converter to determine if catalytic converter performance has deteriorated. Air/fuel ratio compensation keeps the waveform pattern of heated oxygen sensor No. 1 repeatedly switching back and forth from rich to lean. If catalytic converter is functioning properly, the waveform pattern for heated oxygen sensor No. 2 should switch back and forth from rich to lean more slowly than heated oxygen sensor No. 1. If both waveform patterns switch at a similar rate, catalytic converter performance has deteriorated. DTC is set when both oxygen sensors have nearly the same waveform pattern after engine and catalytic converter are at normal operating temperature and vehicle is driven at 55 MPH for at least 5 minutes. Possible causes are:

Exhaust system leak.
Defective heated oxygen sensor.
Defective catalytic converter.