Maintaining of the combine harvesters in an efficient condition throughout the harvest season is an important task of technical service. The most important elements in the work of combine harvesters are their steering mechanisms, the performance of which must have a high degree of availability, especially of their hydraulic drives. The purpose of this study is to increase the operational reliability of the steering drives of the combine harvesters, based on the development of an improved method for diagnosing of their hydraulic drives. During the research, the methods of theoretical and experimental research were used, based on the theory of machine operation, hydraulics, as well as modern methods of experimental studies of hydraulic equipment. The data of the experimental studies were processed by statistical methods using a PC. Theoretically and experimentally, an improved method for diagnosing of the hydraulic drives has been developed with the aim to increase the technical readiness of combine harvesters and reduce the costs of their maintenance and repair. A new methodical approach and results of studies on the creation of a diagnostic system for the hydraulic drive of the combine harvester’s power steering have been developed.