Abstract: In a diagnostic tester for electronic control devices in a variety of different motor vehicles, a universal plug connection is provided that is identical for the various types of motor vehicles. The tester can be connected through the plug connection to a diagnostic plug connection in each motor vehicle that is linked with the control devices to be diagnosed. The connection is standardized mechanically with a fixed number of plug contacts but not in terms of the signal assignments of the plug contacts and the electrical properties of the signals to be transmitted. The universal plug connection is mechanically adapted to the standardized diagnostic plug connection. In addition, an adapter unit is connected to the diagnostic device side of the universal plug connection.