Fail safe device for air/fuel ratio feedback control system

Claim 1

In an air/fuel ratio feedback control system for performing feedback control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, said system comprising: a sensor for detecting the concentration of an exhaust gas ingredient emitted from said engine; an air/fuel ratio control valve having a valve body position thereof disposed to determine the air/fuel ratio of said air/fuel mixture being supplied to said engine; an actuator arranged to displace said air/fuel ratio control valve in a continuous manner in response to an output signal generated by said sensor; and reference position detecting means for generating a first signal when said actuator passes a predetermined reference position provided between two opposite extreme operating positions which mechanically limit a movable range of said actuator, a fail safe device comprising first means for detecting a failure in said air/fuel ratio feedback control system and generating a second signal when said failure is detected; and second means responsive to said second signal to drive said actuator, said second means being adapted to stop said actuator at said predetermined reference position immediately upon being supplied with said first signal while it is driving said actuator, said second means including means for repeatedly driving said actuator over a predetermined operating range inclusive of said predetermined reference position in one direction and in a direction reverse thereto alternately a plurality of times when it is not supplied with said first signal upon said actuator passing said predetermined reference position, and means for driving said actuator from a second predetermined reference position which is one of said extreme operating positions, to a predetermined position and holding te same there when it is not supplied with said first signal even after a predetermined number of times of said repeated driving of said actuator.