| 摘要 |
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 (V 2 ) output from the second air fuel ratio sensor downstream of the catalyst only when the amount of oxygen storage in the catalyst is saturated.
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