The work is aimed to study the change in parameters of traction electric motors of trolleybuses during operation and improvement of the technical state control system, using modern diagnostic methods. Methodology. Solution of the scientific problem is based on the control of technical state and change in the parameters of traction electric motors in the process of operation. For analysis of operational factors, we used the method of mathematical statistics and probability theory. Mathematical modelling and design of experiment, multifactor regression analysis provides an assessment of the reliability of the electric traction motor elements. Physical and statistical methods provide for research and detection of patterns of influence of the factors that accelerate the wear of parts. This allows us to generate models of system reliability that take into account the influence of operational factors on the reliability. Findings. As a result of the studies we obtained quantitative characteristics of the reliability of commutator and established that the failures of traction motors make up 20% of all failures of electrical equipment. We analysed the operation conditions of traction electric motors and determined the failure distribution law N{mx,x}, which allows assessing the progression between gradual failures. We obtained a mathematical model characterizing the object operation. A generalized equation of the failure rate of elements for traction electric motor (TEM) was found. We improved the estimation methods of reliability of TEM element base that makes it possible to determine the ETM lifetime taking into account the features of each trolleybus. We established the failure rate of commutators using physical and statistical methods. Originality. For the first time, the choice of physico-statistical methods of simulation and reliability calculation was substantiated. We determined the regularities of change in parameters of the trolleybus traction electric motor elements, which makes it possible to control the processes of their wear in the operation conditions. We developed a mathematical model for estimating the traction electric motor reliability, based on a system analysis of the probabilities of failures of subsystems to be diagnosed. Practical value. Based on the results of the study, we developed practical recommendations for rational choice of diagnostic parameters of traction motors. Their implementation at the enterprises of electric transport will increase reliability of electric motors in general up to 10%. It is proposed to use the results of the work in the educational process and in the research work of students at the Department of Electric Transport of the O.M. Beketov National University of Urban Economy in Kharkiv. The developed methodology for determination of parameters of traction electric motors when operating trolleybuses allows assessing the reliability of any type of trolleybus traction motor.