Apparatus for controlling air-fuel ratio of internal combustion engine

An apparatus for controlling the air-fuel ratio of an internal combustion engine, comprising:

• an oxygen concentration sensor disposed downstream of a catalytic converter which is disposed in an exhaust passage of an internal combustion engine;
• air-fuel ratio manipulating means for manipulating the air-fuel ratio of an air-fuel mixture to be combusted in said internal combustion engine to converge an output of said oxygen concentration sensor to a target value established depending on an operating state of the internal combustion engine;
• parameter generating means for generating a deterioration evaluating parameter for evaluating a deteriorated state of the catalytic converter according to an algorithm predetermined from data of the output of said oxygen concentration sensor while said air-fuel ratio manipulating means is manipulating the air-fuel ratio of the air-fuel mixture;
• and
• deteriorated state evaluating means for evaluating the deteriorated state of said catalytic converter by comparing the deterioration evaluating parameter with a predetermined decision value;
• said deteriorated state evaluating means comprising:
• means for establishing said decision value depending on an average value of said target value or the output of said oxygen concentration sensor;
• and
• means for evaluating the deteriorated state of said catalytic converter by comparing the established decision value with said deterioration evaluating parameter.