| 摘要 |
<p>PROBLEM TO BE SOLVED: To facilitate diagnosis of a trouble by a method wherein the trouble of a flow passage switching means to switch a first exhaust flow passage having a catalyst through which exhaust gas flows and a second exhaust flow passage having the catalyst which exhaust gas bypasses is diagnosed through the catalyst based on an air-fuel ratio of exhaust gas and an air-flow ratio in a spot situated down stream from a bypass passage confluent part. SOLUTION: First and second catalysts 18 and 20 are located in series in spots situated upper stream and downstream from of an exhaust passage 9 and a bypass passage 9b bypassing the catalyst 18 is connected to the exhaust passage 9, and a switching valve 16 is arranged at a branch part where the bypass passage 9b is branched from an exhaust passage 9. O2 sensors 19 and 21 are arranged in a spot right upper stream from the catalyst 18 of the exhaust passage 9 and a spot situated upper stream from the catalyst 20 and downstream from a confluence part of the bypass passage 9b. Since, during the use of the bypass passage, the fluctuation width of an air-fuel ratio is reduced and the inversion period T1 of the output of the O2 sensor 19 is increased to a value higher than the inversion period T2 of the output of the O2 sensor 20, when T2 is higher than a product of T1 and a given value K, it is decided that trouble occurs to the switching valve 16 and an alarm is generated.</p> |
