In order to permit diagnosis of operation and function parameters in a motor vehicle which is equipped with an electronic microprocessor control system without the necessity of providing in the diagnostic equipment substantial additional memory capability, the control system includes a sub-program for diagnosis. A first subprogram executed by the control system in response to a command from the diagnostic equipment gives the appropriate actual operational data to the diagnostic equipment. A second subprogram simulates nominal values for specific vehicle data under various operating conditions and compares these with the actual values. The resulting information is given to an operator by a display and can additionally include various instructions, e.g. repair instructions or a trouble shooting sequence based on the diagnosis.