Patent ID: 7146972

Claim:
An air fuel ratio control system comprising: an exhaust gas sensor disposed in an exhaust passage of an internal combustion engine and arranged to sense an oxygen concentration; an actuator arranged to perform an air fuel ratio control in accordance with the oxygen concentration sensed by the exhaust gas sensor; and a controller 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 statue; wherein the controller is configured to judge the exhaust gas sensor to be in the active state when the output of the exhaust gas sensor continues to be smaller than or equal to the first predetermined value for a time longer than or equal to a predetermined time length.