Closed Loop Mode
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.
When HO2S reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has passed since engine start-up, PCM operates in closed loop mode. In closed loop mode, PCM controls air/fuel ratio based upon HO2S signals (in addition to other input parameters) to maintain as close to a 14.7:1 air/fuel ratio as possible. If HO2S cools off (due to excessive idling) or a fault occurs in HO2S circuit, vehicle will re-enter open loop mode.