Patent ID: 8521405

Claim:
An air-fuel ratio diagnostic device for an internal combustion engine that includes an exhaust manifold provided with a junction in which exhaust discharged from a plurality of cylinders meet, the device comprising: an air-fuel ratio sensor that detects oxygen concentration in the exhaust at the junction; and a determination unit that performs a determination related with a variation between the cylinders, which includes a variation in an air-fuel ratio between the cylinders, based on a detection value of the air-fuel ratio sensor, wherein a change amount in the detection value for a certain time when the detection value is changing from a peak value at the lean side toward a peak value at the rich side is defined as a rich change rate, a change amount in the detection value for a certain time when the detection value is changing from a peak value at the rich side toward a peak value at the lean side is defined as a lean change rate, and the determination unit determines a degree of variation between the cylinders based on the rich change rate and the lean change rate.