Closed Loop
WARNING: This page does not describe the selected car, but rather 10 other vehicles, including the 1994 GMC Yukon, 1994 GMC Suburban, 1994 GMC Pickup, 1994 GMC Cab & Chassis, and 1994 GMC C3500 HD. However, it is still accessible from the selected car via links, so may be relevant.
When O2S sensor 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. In closed loop, PCM controls air/fuel ratio based upon O2S sensor signals (in addition to other input parameters) to maintain as close to a 14.7:1 air/fuel ratio as possible. If O2S sensor cools off (due to excessive idling) or a fault occurs in O2S sensor circuit, vehicle once again enters open loop mode.
On most engines, oxygen sensor is equipped with an internal heating element. This element enables system to reach "closed loop" sooner and maintain closed loop even during periods of extended idle.