The article presents the results of a study of the influence of operating conditions and modes on the technical condition of grain harvesters. The grain harvester gradually loses its performance during operation. To ensure stable operation of the fleet of agricultural machines, maintain its technical condition and increase operational reliability, technical control of grain harvesters is carried out. The analysis of the technical condition of combine harvesters showed that there remains a high percentage of defective combine harvesters, downtime for all types of repairs and the number of unplanned repairs. One of the main reasons for this condition is the low level of organization and quality of technical control, insufficient mechanization of technological processes. The basic element of the technological preparation of repair production is the development of a technological process that ensures the availability of a complete set of technological documentation at the enterprise for organizing and carrying out technical maintenance and repairs of traction rolling stock. During the development of technological processes of repair, the selection of technological operations and means of their technological equipment, normative documents of requirements for tolerances, clearances, forces of landings, pressings, threaded connections, processing quality, assembly accuracy, etc. is carried out. Technical conditions for the development of non-standard technological equipment are determined from the developed technological operations. As a target function, the rational periodicity of maintenance of grain harvesters was taken, namely, the rational time between services. The analysis of the results of known works on mathematical modeling and periodicity of repair and maintenance of complex technical systems based on Markov and semi-Markov processes allowed us to propose complex criteria as criteria for modeling the objective function: the functional of readiness, which is used to estimate the allowable time, and the functional of technical use, which allows determining the value of the optimal time between services of the combine harvester according to the technical parameter at the maximum point.