With the extremely advanced and complex design of modern vehicles, their high power and torque, it is necessary to employ advanced diagnostic systems. Quick detection and diagnosis of all defects will protect the vehicle against more serious failures, and will prevent excessive fuel consumption and harmful emissions, as well as prevent the reduction of the efficiency and functionality of vehicles. The mechatronic vehicle diagnostic system must include the diagnostic modules for the following components: engine, powertrain, steering, brake system, suspension and electrical equipment. The study discusses the hardware and software components of the mechatronic vehicle diagnostic system, and presents the sample sensors installed in the vehicle to monitor the parameters of components included in the diagnostic system. The vehicle diagnostics system has been presented using the example of diagnostics of wheeled tractors. For this purpose, two alternative diagnostic devices have been developed: a prototype MSDC-1 device, and a production version of the MSDC-2 device. Following a minor adaptation, both devices can be used in the diagnostics of other wheeled vehicles powered by I.C. engine. In further paragraphs of the study, some of the field tests performed on the diagnostic units are discussed. The study also presents some of the test results in the form of diagrams of changes in the temperature of specific engine components. In the diagrams illustrating the results of the tests, the changes of specific operating parameters of the wheeled tractor over time during tests of the MSDC-1 and MSDC-2 units can be observed. The devices differ significantly in terms of design, but they share a purpose, namely online diagnostics of vehicle condition
