Closed Loop Mode
WARNING: This page is about a different car, the 2003 Saturn LW300 and 2003 Saturn L300. However, it is still accessible from the selected car via links, so may be relevant.
When O2S reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has passed since engine start-up, ECM operates in closed loop mode. In closed loop mode, ECM controls air/fuel ratio based upon O2S signals (in addition to other input parameters) to maintain as close to a 14.7:1 air/fuel ratio as possible. If O2S cools off (due to excessive idling) or a fault occurs in O2S circuit, vehicle will re-enter open loop mode.
A Heated Oxygen Sensor (HO2S) is equipped with an internal heating element. The heating element enables system to reach and maintain closed loop mode sooner, even during periods of extended idle.