Patent ID: 7240479

Claim:
An abnormality diagnosis apparatus for an internal combustion engine comprising: a variety of kinds of sensors that detect an operating condition of said internal combustion engine having an exhaust system; a first air fuel ratio sensor and a second air fuel ratio sensor that are arranged at locations upstream and downstream of a catalyst arranged in said exhaust system for detecting the concentration of a specific component in an exhaust gas; a first comparison section that compares a first output signal value of said first air fuel ratio sensor with a first comparison value; a second comparison section that compares a second output signal value of said second air fuel ratio sensor with a second comparison value; a comparison value calculation section that calculates said first comparison value in accordance with a comparison result of said second comparison section; an air fuel ratio correction amount calculation section that calculates an air fuel ratio correction amount corresponding to a comparison result of said first comparison section; a feedback control section that controls an amount of fuel supplied to said internal combustion engine in accordance with said air fuel ratio correction amount in a feedback manner; a feedback operation state determination section that determines whether said engine operating state is a feedback operation state in which said feedback control section is operated; an averaging section that averages said second output signal value during operation of said comparison value calculation section; an oxygen storage saturation determination section that determines whether an amount of oxygen storage in said catalyst is in a saturated state; an averaging start determination section that activates said averaging section only when said oxygen storage saturation determination section determines that the amount of oxygen storage in said catalyst is saturated; and a fuel control system abnormality determination section that determines the presence of abnormality in said fuel control system of said internal combustion engine when the air fuel ratio average value calculated by said averaging section becomes out of a predetermined range; wherein said oxygen storage saturation determination section calculates a time of execution of feedback control elapsed after a determination result of said feedback operation state determination section has changed from a non-feedback operation state into a feedback operation state, and determines that the amount of oxygen storage in said catalyst is saturated when said elapsed time reaches a predetermined time or more at which the amount of oxygen storage in said catalyst becomes a predetermined value.