The features and the reasons of interval faults caused by microcomputercontrolled system in vehicles,which were called as "difficult and complicated cases" in auto repair trades were introduced.The component,function and applications of the interval fault analyzer were explained.By using the instrument,the faults could be located in minimal range in shorter time among a relative larger possible fault range.The instrument which is small,easily installed with a low cost,can work in conditions when the vehicles are in motion.The operation of the instrument is easy to study.It is easily popularized.