Abstract: The study presents a method for synthesizing a system for identification of faults that appear in various elements of complex mechatronic systems described by nonlinear differential equations. This method is based on the use of diagnostic observers and the logical-dynamic approach which makes it possible to apply linear methods in the diagnosis of faults in nonlinear systems as well as introducing special feedback by residual signal to the observers which provides an accurate estimation of the values of the emerging faults in real time. On the example of identification of faults arising in electric drives of a multi-link manipulator, the operability and high efficiency of the proposed method are shown.