Closed Loop

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.