A diagnostic device and a diagnostic method for use with a bus system that includes stations interconnected over a bus, one station of which is set up as the master station and the other stations being slave stations. A diagnosis operation is performed which allows differentiated fault information to be obtained with regard to individual outputs of the stations, where the expenditure for the diagnostic device is low. The diagnosis is achieved by providing a diagnostic device only in the master station, by storing voltage and/or current information on at least one output of at least one slave station to be diagnosed in the diagnostic device, and by performing the diagnosis of the outputs under preselected or preselectable conditions on the basis of a program stored in the master station.