This document presents a new method to detect sensor node failure or malfunctioning in wireless sensor networks (WSNs). It uses round trip delay (RTD) time to estimate confidence factors of RTD paths. Based on the confidence factors, failed or malfunctioning sensor nodes can be detected. The method was tested using a hardware simulation with four microcontroller-based sensor nodes communicating over Zigbee. The results showed that sensor node failures could be accurately detected by analyzing changes in instantaneous RTD times and confidence factors stored in a lookup table.