To surely diagnose leakage when closing a flow passage switching valve for switching a main passage and a bypass passage.
As an exhaust emission control device, a small bypass catalytic converter is arranged in the bypass passage parallel to a part of the main passage having a main catalytic converter on the downstream side, and the flow passage switching valve opens and closes the main passage. Air-fuel ratio sensors are respectively provided on the upstream side and the downstream side of the main catalytic converter and the upstream side and the downstream side of the bypass catalytic converter. A leakage diagnosis of the flow passage switching valve is performed by changing the target air-fuel ratio stepwise to lean and rich in a closing position. The exhaust air-fuel ratio on the upstream side of the bypass catalytic converter, immediately becomes rich, but becomes rich by delaying when there is no leakage on the upstream side of the main catalytic converter, and becomes richer by influence of leaked exhaust gas when there is leakage, and is discriminated from a difference in the average air-fuel ratio between both of a period T.
COPYRIGHT: (C)2007,JPO&INPIT
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