System for diagnosing degradation of air-fuel sensor

A system for diagnosing degradation of an air-fuel ratio sensor disposed in an exhaust passage of an internal combustion engine, the system comprising:

• a diagnosis air-fuel ratio control means for conducting a diagnosis air-fuel control in which an air-fuel ratio correcting amount is corrected in a direction away from a diagnosis target air-fuel ratio for diagnosis by a predetermined skip amount which is greater than a predetermined integral amount when a detected air fuel ratio is corrected toward the diagnosis target air-fuel ratio by the predetermined integral amount to exceed the diagnosis target air-fuel ratio;
• a time constant computing means for computing a time constant of the air-fuel ratio sensor based on the detected air-fuel ratio during the diagnosis air-fuel control;
• a time delay computing means for computing a time delay of the air-fuel ratio sensor based on the detected air-fuel ratio during the diagnosis air-fuel control;
• and
• a diagnosis means for diagnosing at least one of a rich-side time constant degradation, a lean-side time constant degradation, a rich-side time delay degradation, and a lean-side time delay degradation of the air-fuel ratio sensor based on a time constant information computed by the time constant computing means and a time delay information computed by the time delay computing means.