Conditions For Setting DTC

WARNING: This page is about a different car, the 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, 2003 Chevrolet Monte Carlo, and 2003 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.

The PCM detects that the HO2S 1 voltage switched from rich-to-lean fewer than 20 times within a 100 ms monitoring period.
The PCM detects that the HO2S 1 voltage switched from lean-to-rich fewer than 20 times within a 100 ms monitoring period.