The invention relates to a diagnostic system, which uses a radio-based communications interface to download the results of the on-board system diagnosis in the vehicle and evaluates said results on an off-board diagnostic platform. The on-board diagnosis collects vehicle data by interrogating buses, to which the control units are connected, to identify errors. Said errors are processed and stored in a memory, together with relevant information concerning the condition of the control units. A diagnostic computer can retrieve said information at defined intervals and store it in a ring buffer. Once the telediagnosis has been initiated, the most significant data is sent to the diagnostic center at the customer assistance center (CAC). The data is then evaluated in the CAC on a central diagnostic platform using a complex diagnostic program. With the aid of the diagnostic program, conclusions are drawn with regard to the cause of the error. If additional vehicle data is required, it can be subsequently requested. Once the subsequently requested data has been received, the diagnostic program resumes and the analysis result is improved. The subsequently received data is sent to the center. The subsequent request for data can occur as often as required and is based on a configuration file that can be freely supplied with data or modified, said file being evaluated whilst the telediagnosis is in progress. The analysis results of the diagnostic program are converted from the vehicle-specific data format used by the control units into an XML metaformat and are stored.