| 摘要 |
<p>PURPOSE: To prevent erroneous judgment due to the deterioration of an upstream side air-fuel ratio sensor by setting a judgment reference value based on the control frequency of air-fuel ratio feedback control when a judgment is made on catalyst deterioration by comparing the reversal condition of an upstream side air-fuel ratio sensor to a downstream side air-fuel ratio sensor for a catalyst converter with the judgment reference value. CONSTITUTION: A catalyst converter 27 is disposed in an exhaust path 23, an upstream side O2 sensor 28 is disposed at the upstream side of the catalyst converter 27, and a downstream side O2 sensor is disposed at the downstream side of the converter respectively. These O2 sensors 28 and 29 acting as an air-fuel ratio sensor generate electro motive force in response to remaining oxygen concentration in exhaust gas, and electro motive force is rapidly changed along the boundary of a theoretical air-fuel ratio in particular. Besides, when the upstream side O2 sensor 28 is deterirated, the control period of air-fuel ratio feedback control is lengthened. When catalyst deterioration is diagnosed by using the reversal frequency ratio of the O2 sensor 28 to the O2 sensor 29, since a judgment reference value is set while the lowering of control frequency of air-fuel ratio feedback control is taken into consideration, deterioration can be diagnosed with high accuracy.</p> |
