in an air conditioning system for an automobile the indoor - outdoor air change - over door is controlled by a noxious gas sensor in the outdoor air . the output signal of the gas sensor is compared by a microcomputer to a reference signal which is determined by the minimum value of the output signal of the gas sensor produced from the time when the gas sensor is energized to the time when it is deenergized . the microcomputer outputs a signal causing an actuator to move the change - over door from the outdoor air introduction mode to the indoor air recirculation mode when the difference between the output signal of the gas sensor and the reference level exceeds a predetermined amount .