Patent attributes
In a system that executes air fuel ratio feedback control based on air fuel ratio signals of a pair of air fuel ratio sensors arranged at locations upstream and downstream of a catalyst of an internal combustion engine, an abnormality diagnosis apparatus can diagnose abnormality in a fuel system of the internal combustion engine in an accurate manner by using a signal of the air fuel ratio sensor downstream of the catalyst. A control unit in the form of an ECU (30) includes a feedback operation state determination section (32), an oxygen storage saturation determination section (33), an averaging start determination section (34), an averaging section (35), and an abnormality determination section (36). An abnormality in the fuel system of the internal combustion engine is determined by using a second air fuel ratio signal (V2) output from the second air fuel ratio sensor downstream of the catalyst only when the amount of oxygen storage in the catalyst is saturated.
