| 摘要 |
An air fuel ratio control apparatus for an internal combustion engine can improve accuracy in catalyst degradation diagnosis. The apparatus includes a first air fuel ratio feedback control section that adjusts the air fuel ratio of a mixture supplied to an engine in accordance with an output of an upstream air fuel ratio sensor and a predetermined control constant thereby to make the air fuel ratio periodically oscillate in rich and lean directions, an average air fuel ratio oscillation section that operates the control constant based on an amount of oxygen occlusion of a catalyst so that an average air fuel ratio obtained by averaging the periodically oscillating air fuel ratio is caused to oscillate in the rich and lean directions, and a catalyst degradation diagnosis section that diagnoses catalyst degradation based on correlation between the oscillation of the average air fuel ratio and an output of the downstream oxygen sensor.
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