Abstract: A diagnostic apparatus of an assist air supply system of an engine, in which an AAI solenoid valve is provided in an assist air passage and when diagnosing, a difference between a maximum value of intake manifold pressure when the AAI solenoid valve opens and a minimum value of intake manifold pressure when the AAI solenoid valve closes is calculated. When the calculated difference is smaller than a specified threshold value, it is judged that there is something abnormal in the assist air supply system.

Abstract: In an abnormality detecting device and method for an air-fuel ratio control system of an internal combustion engine, first discriminating whether a renewing number NLR of grids in a map of air-fuel ratio learning control is larger than a set value or not. If the number is larger than the set value, the difference between the number and previously renewed learning values is calculated. Then, judging whether the difference is larger than a predetermined value or not. If the difference is larger than the predetermined value, it is judged that an abnormality occurs in an intake air measurement system or a fuel injection system of an air-fuel ratio control system. Accordingly, the abnormality in the systems is distinguished from the deterioration due to normal aging.