INTERNAL-COMBUSTION-ENGINE DIAGNOSTIC DEVICE

摘要

This internal combustion engine is equipped with an exhaust purification catalyst (20) and an air-fuel-ratio sensor (41) on the downstream side of the exhaust-purification catalyst, and executes a fuel-cut control for stopping the supply of fuel, and a post-recovery rich control for controlling, after completing the fuel-cut control, the exhaust air-fuel ratio so as to be a rich air-fuel ratio. On the basis of the output air-fuel ratio outputted from the air-fuel-ratio sensor, first air-fuel-ratio-change properties and second air-fuel-ratio-change properties are calculated at the times when, after completing the fuel-cut control, the output air-fuel ratio first passes through a first air-fuel-ratio region (X) and a second air-fuel-ratio region (Y) which differs therefrom. The diagnostic device determines the state of the air-fuel-ratio sensor to be normal, abnormal, or determination-pending on the basis of the first air-fuel-ratio-change properties, and when the state is determined to be unclear, determines the state of the air-fuel-ratio sensor to be normal or abnormal on the basis of the second air-fuel-ratio-change properties. As a result, it is possible to minimize the impact of changes to the state of the exhaust purification catalyst, and accurately diagnose a responsiveness-deterioration abnormality of the downstream-side air-fuel-ratio sensor.