| 摘要 |
<p>A deterioration detecting method for a catalyst in an exhaust system of an internal combustion engine (1) for purifying exhaust gases, the system including a first oxygen concentration sensor (17) and a second oxygen concentration sensor (18) disposed, respectively, upstream and downstream of the catalyst (14a). Intake air flow rate of the engine is detected (7). An amount of oxygen flowing into the catalyst (14a) is calculated according to an oxygen concentration detected by the first sensor (17) and the detected intake air flow rate. An air-fuel ratio of an air-fuel mixture supplied to the engine (1) is controlled by alternately switching between lean and rich with respect to the stoichiometric ratio according to a result of comparison between the calculated and target values of the inflowing oxygen amount. A deterioration degree of the catalyst (14a) is detected based on an output of the second sensor (18) when performing the switching control of the air-fuel ratio.
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