In diagnostic expert systems we utilize the professional knowledge accumulated in the given field. This is processed by an adequate software. Although, it causes problem that the regularities of the diagnostic systems cannot always be algorithmized. In the forthcoming section I analyse the connection between the elaboration of diagnostic procedures used for the operation of vehicles and vehicle systems and their modelling from a different perspective. It can provide an aid for the elaboration of a new diagnostic procedure if we apply models for the analysis of the behaviour of the phenomena we would like to study [10, 11].