It was shown previously that four tests are required in order to detect single fault and to locate single link stuck fault for a class of multistage interconnection networks. In this paper, we show that only three tests are necessary and sufficient both to detect single fault and to locate single link stuck fault. A fault diagnosis method with four valid state for a class of multistage kinterconnection networks is presented. Using this method, all single link stuck fault or all single switching element faults, can be detected by at most 4 log2N+e tests. In the example, the location and type of single fault are pinpointed, and unlocatable probabilities of questionable of questionable regions are given.