In practical applications, objects with directional (system) and non-directional (chain) signal propagation can be distinguished as typical objects of diagnosis. The first include, for example, continuous control systems, to the second - electrical circuits. To date, not fully solved problem, which has important applied applications, it is necessary to consider, in particular, the diagnosis of systems with limited access to their internal points. Methods for diagnosing continuous inertial-free systems for a wide range of faults are considered with allowance for limited access to the internal points of the diagnosed systems. Diagnosed malfunctions are presented as parametric, that is, leading only to changes in the parameters of the systems under study and structural ones, which change some part of the object (subsystem) in an arbitrarily arbitrary manner.Definitions are formulated that make it possible to distinguish (distinguish) systems based on the properties of observables and controllability. At the same time, problems of fault location in classes of subsystems with independent supervision, subsystems with independent control and subsystems with independent supervision and control were investigated. The criteria for the distinguish ability of subsystems, to the accuracy of which the fault location is established, are given.