This paper presents a fault tolerant controller for asynchronous sequential machines subject to unauthorized state transitions caused by adversarial inputs. The input/output machines are considered where direct access to the machine's state is unavailable. If the asynchronous machine satisfies some condition on transition feature and reachability, we can design a simple output feedback controller that needs neither a state observer nor output bursts. Despite insufficient knowledge of the current state, the proposed controller automatically counteracts any transient fault so that the closed-loop system can maintain the normal input/output behavior. (C) 2014 Elsevier Ltd. All rights reserved.