Closed Loop

When O2S reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has passed since engine start-up, PCM/VCM operates in "closed loop" mode. In "closed loop" mode, PCM/VCM 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.