The achievement of technical progress in the operation of machines is becoming increasingly relevant in terms of economy. In this matter, the reliability of the machine and its individual components and details plays a decisive role. The latter is an important factor in the smooth operation of machines, which is associated with one of the main factors of reliability - reliability. During the operation of agricultural machinery, there are more or less oscillations, vibrations and other aggressive phenomena in mechanical joints, which affect the technical and economic indicators of machines. A similar effect on the operation of machines is exerted by air pressure, which changes at a certain height above sea level, air humidity and temperature, high soil resistivity, compaction, weediness, crops grown on slopes, etc. It is important that the fault is characterized by a probabilistic-statistical nature and is subject to the elements of statistical research. The fault can lead to a forced stop of the process, to carry out the necessary services, the duration of which depends on the complexity of the technical service, but it is difficult to predict the fault in advance. The theory of reliability of machines and their systems and mechanisms has made a significant contribution to the improvement of the field of mechanical engineering, in the form of so-called technical service and operational repairs. The paper discusses the relationship of the elements of probability theory to engine power and unit reliability in general.