A fault detection and isolation methodology has been developed for validation of sensors and plant components. Significantly, the isolation of most consistent and inconsistent subsets of measurements for the purposes of estimation and failure detection is performed on the basis of a multi-level, as opposed to the usual bi-level, fail/no fail, characterization of the inconsistencies among measurements. This is achieved by the concurrent checking of the relative consistency of smaller size subsets of measurements. The algorithm has been computer-coded for real time applications, and validated by on-line demonstration in an operating nuclear reactor.