Conditions for Setting the DTC
WARNING: This page is about a different car, the 2004 Pontiac Sunfire and 2004 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
The PCM detects that the difference between the HO2S 1 rich-to-lean average transition time and the lean-to-rich transition time is more than a calibrated value.