Exhaust gas sensor activation judgment and air fuel ratio control system/method

An air fuel ratio control system includes an exhaust gas sensor to sense an oxygen concentration in an exhaust passage of an internal combustion engine, and a controller. The controller is configured to judge that the exhaust gas sensor is in an active state, by monitoring a decrease of an output of the exhaust gas sensor from a level greater than a first predetermined value, to a level smaller than or equal to the predetermined first predetermined value, and to allow the air fuel ratio control when the exhaust gas sensor is judged to be in the active state.