For a new class of fault situations called hybrid fault situations, we consider the fault diagnosing capabilities of systems which for testing/monitoring purposes can be viewed as being composed of independent units. The classical Preparata, Metze, and Chien model (PMC model) is used to specify the various testing assignments among the units. Hybrid fault situations are described as explicitly bounded combinations of permanently and intermittently faulty units. This new concept of a hybrid fault situation includes as special cases the all permanent fault case and the unrestricted intermittent fault case which have both been previously considered with PMC models. A general characterization of the so-called connection assignment of a PMC model is established for a diagnosing capability which is referred to as hybrid fault diagnosability without repair. This diagnosing capability is compatible with the well-known permanent fault diagnosability without repair concept, and the quality of this diagnosing capability is shown to be correct, but sometimes an incomplete diagnosis where the incompleteness is solely a consequence of intermittently faulty units. The characterization for hybrid fault diagnosability without repair is seen to encompass as extreme cases the previously known special characterizations for permanent and unrestricted intermittent diagnosability without repair. Fundamental interrelationships are determined among parameter bounds used to describe the hybrid fault cases which can be diagnosed for a given PMC model.

Diagnosable systems composed of interconnected units which are capable of testing each other have been studied primarily from the point of view of permanent faults. Along such lines, designs have been proposed, and necessary and sufficient conditions for the diagnosis of such faults have been established. In this paper, we study the intermittent fault diagnosis capabilities of such systems. Necessary and sufficient conditions are derived and bounds are established for some well-known systems. It is seen that in contrast to permanent fault diagnosable systems, there exists only a single type of intermittent fault diagnosable system. A procedure is given to determine the intermittent fault diagnosability of any given system.