Abstract—In this paper, we investigate fault diagnosis of composite asynchronous sequential machines with cascade composition. An adversarial input can infiltrate one of two submachines comprising the composite asynchronous machine, causing an unauthorized state transition. The main objective is to specify the condition under which the controller can diagnose any fault occurrence. Two control configurations, state feedback and output feedback, are considered in this paper. In the case of output feedback, the exact estimation of the state of the front machine is impossible since the current state is inaccessible. Due to feature of cascade composition, we must consider the case where the rear submachine undergoes a faulty transition caused by the adversarial input occurring to the front submachine. Fault detectability is also addressed in the case of output feedback.