A wireless sensor network (WSN) is a wireless network consisting of spatially distributed autonomous devices using sensor nodes in a wide range of applications. However, the reliability of a WSN is an important issue in the support of the applications. To achieve this goal, the fault tolerance of WSNs must be studied. For underlying malicious failure characteristics on nodes in a WSN, a fault diagnosis agreement (FDA) is proposed to make each fault-free node detect/locate a common set of faulty nodes by collecting the accumulated messages. However, the proposed protocol can cause each fault-free node to obtain a maximum number of a common set of tolerable faulty nodes. Therefore, the proposed protocol can enlarge the fault tolerance capability by allowing malicious faults to exist in a WSN.