| 摘要 |
<p>PURPOSE:To secure diagnosis opportunity of catalyst deterioration by comparing output fluctuation frequencies of first and second sensors detected by an output fluctuation frequency detection means, and judging deterioration of exhaust purifying catalyst, even when control points are different at upstream and downstream sides of the catalyst. CONSTITUTION:An output fluctuation range detection means 12 detects an output fluctuation range of a second oxygen sensor 17 on a downstream side. The air-fuel ratio control property correction means 12 corrects air-fuel ratio feedback control by the air-fuel ratio feedback control means 12 for making the output fluctuation range of the second oxygen sensor 17 on the downstream side approach a specified output range including an output level corresponding to a target air-fuel ratio. A fluctuation frequency is detected by the output fluctuation frequency detection means 12, which frequency has a boundary composed of target air-fuel ratio corresponding values of outputs of upstream and downstream oxygen sensors 16, 17. The output fluctuation frequencies of the first and second oxygen sensors 16, 17 are compared to each other, and deterioration of an exhaust emission catalyst 10 is judged by the catalyst deterioration diagnosis means 12.</p> |
