| 摘要 |
PROBLEM TO BE SOLVED: To avoid a wrong diagnosis by an oxygen sensor, by detecting a parameter indicating an output response of the oxygen sensor, comparing the parameter with a parameter judgment value set based on an activity degree and a deterioration degree of a catalyst, and diagnosing a response deterioration of the sensor. SOLUTION: A control unit 12 detects an output inverse frequency of an oxygen sensor 17 as a parameter indicating an output response when a diagnosis permission condition is satisfied. Then, a parameter judgment value is set from a catalyst activity degree estimated from a catalyst temperature and a catalyst deterioration degree obtained by comparing output inverse frequencies of oxygen sensors 16, 17. The output inverse frequency of the sensor 17 is compared with the parameter judgment value. Only a response change of the sensor 17 is detected irrespective of a change of an air-fuel ratio change speed at the downstream side. In other words, when the output inverse frequency is smaller than the judgment value, it is detected as the response deterioration of the sensor 17. Accordingly, the output response of the sensor 17 is prevented from being diagnosed wrong. |
