Abstract: A diagnostic test system having a portable testing device for motor vehicles, controllers having an on-board diagnostic device being provided which control and monitor the engine control system and other systems of the vehicle, generate and store error codes, and which can be connected to an external testing device via a motor vehicle-side diagnosis/test plug. The external portable testing device is equipped with a modem and connected to a mobile telephone, a hand-held cell phone in particular, in order to perform error diagnosis and troubleshooting in such an interconnected network outside stationary repair shops.