Patent ID: 8744729

Claim:
An apparatus for detecting abnormal air-fuel ratio variation among cylinders of a multi-cylinder internal combustion engine, comprising: a catalyst element that is provided in an exhaust passage of the multi-cylinder internal combustion engine, and that oxidizes at least hydrogen contained in exhaust gas to remove the hydrogen; a first air-fuel ratio sensor that detects a first exhaust gas air-fuel ratio which is an air-fuel ratio of exhaust gas that has not passed through the catalyst element; a second air-fuel ratio sensor that detects a second exhaust gas air-fuel ratio which is an air-fuel ratio of exhaust gas that has passed through the catalyst element; and an abnormality determination unit that determines whether abnormal air-fuel ratio variation among the cylinders has occurred based on an amount by which a detection value of the second exhaust gas air-fuel ratio is leaner than a detection value of the first exhaust gas air-fuel ratio.