The functioning of complex man-machine systems (CMMS) (for example, aviation-transport systems (ATS), networks of railways, atomic power stations, oil pipelines, etc.) is formed by interrelations and interactions of various processes: command-information process; process of actions of executors (operators, crew, dispatchers, etc.); process of functioning of equipment; power supply process; process of maintenance by raw materials, accessories, cargo, passengers, etc.; process of interaction with the environment, etc. Traditional substantive provisions, models and methods of technical diagnosing of small objects are not effective for complex (large-scale) man-machine systems. This paper contributes to the studies of new models and methods for identification of the reasons of failures in such systems.