Reliability of the methods for diagnosing vehicles by operator-diagnostician has been studied. The methods are aimed at fast and effective detection of malfunction with the minimum use of technical means to reduce complexity and increase the efficiency of diagnosing. The paper reveals the intellectual abilities of human in diagnosing vehicles; demonstrates the advantages and disadvantages of intelligent diagnostic systems in comparison with technical diagnostic systems; describes what knowledge is needed for diagnostics and what operational and technical documentation helps increase the efficiency of intelligent vehicle diagnostics; presents the algorithm of intelligent diagnostics of vehicles. It is proved that a highly qualified diagnostician can make vehicle diagnostics much less complicated and more efficient. The results of this work are intended by drivers, maintenance workers, developers of operational and technical documentation, and implementors involved in the improvement of technical means of diagnostics.