| 摘要 |
<p>PROBLEM TO BE SOLVED: To improve the degradation diagnostic performance of an exhaust purifying catalyst by diagnosing the activation of a catalyst at the time of the decrease speed becoming the specified value or less in association with the decrease of a reversing cycle ratio of an air-fuel ratio detecting means on the lower reaches, and diagnosing the degradation of the catalyst on the basis of the reversing cycle ratio. SOLUTION: During the operation of an internal combustion engine 1, a C/U 6 measures the rich-lean reversing cycles of an upstream and a downstream air-fuel ratio sensors 10, 11 of an exhaust purifying catalyst 9 so as to compute a reversing cycle ratio and the decrease speed per unit time of the reversing cycle ratio. Whether or not the decrease speed is the specified value or more is then judged, and in the case of the specified value or more, after waiting the decrease speed becoming almost zero, whether or not the reversing cycle ratio is changed to the increase side at a specified increase rate or more is judged. In the negative case, whether or not the reversing cycle ratio is the specified value or less is judged after the lapse of specified time. In the affirmative case, the catalyst 9 is diagnosed to be normal. In the negative case, the catalyst 9 is diagnosed to be degraded.</p> |
