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The problem of increasing the dependability of the engine, which is the most complex and expensive unit of an automotive vehicle, cannot be solved without objective and reliable information on the failures and malfunctions of its components, their causes, actual life, as well as the factors affecting such indicators in real operational conditions. Manufacturing factories do not always have such information, hence design deficiency failures associated with design and development flaws are among the most common causes of loss of engine operability.The aim of this paper is to study the engine operational dependability using the results of their maintenance and repair.The methods are based on operational tests of engines that yield the most complete and objective information on their dependability, as they were conducted in typical operational conditions of automobile operating companies in the course of vehicle maintenance and repair. The results of the studies processed with the standard Statistica 6.0 are represented in the form the statistical evaluations of the dependability of primary structural engine components (times to failure, changes in the probability of nofailure depending on the travelled distance). The analysis of the obtained information allows estimating the level of actual dependability of the engine, identifying design flaws, developing specific measures aiming to increase operational dependability. Information obtained during such tests is useful not only to the engine manufacturers, but to the operators as well, as it enables a scientific substantiation of the norms of operability. For the purpose of identification and localization in the process of maintenance and repair of specific engine malfunctions, the paper substantiates a set of diagnostic parameters and their standard values.Conclusions. The research allowed elaborating a set of diagnostic parameters for evaluation of the technical condition of primary engine systems (cylinder-piston group, crank and gas distributing mechanisms) that define and limit its dependability. The application of the findings in the automobile maintenance and repair processes enables a significant improvement of the engines’ operational dependability and reduction of the costs of ensuring their operability.