A personal computer (PC) is connected to the MUTIII unit and used as a system's control terminal, and communicates with a vehicle ECU through the VCI. In addition to being able to handle all diagnosis functions covered by the conventional MUT-II tester, the MUT-III system can display data as easy-to-understand diagrams and graphs on a large screen of the PC. Unlike the MUT-II, the MUT-III can retain data on all vehicles, from old to new models, classified according to destination in the PC's hard disk. This eliminates the awkward but essential job with the MUT-II, of switching the data list (communications protocol data and diagnosis item list database) by replacing the ROM pack with another according to the vehicle model and year model.