The article is dedicated to various questions of control and diagnostics of automobile technical conditions by applying on-board diagnostic system. The structure of automated diagnostic system was analyzed. Various types of sensors, signals and required types of their transfigurations were enlisted . The relationship between the structural and diagnostic parameters of the car is given, where all systems and car units are divided into groups. The first group consists of systems, units and machine parts that provide traffic safety, the second group contains the remaining functional systems. A structural and investigation scheme for diagnosing automobile’s brake gear with an air pressure brake system was developed.