Closed Loop Mode

When O2S 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 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.

On most engines, O2S is equipped with an internal heating element. This type of sensor is known as a Heated Oxygen Sensor (HO2S). The heating element enables system to reach and maintain closed loop mode sooner, even during periods of extended idle.