A diagnostic platform adapted for coupling to a scanner to download data from vehicle computers. The system stores information regarding vehicle identifications, drivability symptoms, vehicle system and component tests and service codes which can be registered by the vehicle on-board computer. System software permits user input of vehicle identification and, in one mode, displays a fault library from which the user can select, whereupon the system selects from the library tests pertinent to diagnosing causes of selected faults and displays them in a hierarchically ranked order based on likelihood of success. The user can then initiate any displayed test. In other modes, the system initially displays one of the libraries of system or component tests, from which the user selects, whereupon the system highlights icons which can be selected for initiating pertinent test procedures. Selected test procedures include links to engine analyzer or scanner hardware or other appropriate test modules.