Abstract: An interactive diagnostic system is disclosed herein for use with an automotive vehicle of the type including a network of sensors and actuators for independently sensing and actuating a number of different functions within the vehicle and an onboard computer for monitoring the sensors and controlling the operation of the actuators. This system provides the automotive service professional with all of the tools necessary to provide precision diagnostic testing on todays computer-controlled cars. This is accomplished by providing the system with means including an external computer for controlling operation of one or more specific actuators independent of the onboard computer and for simulating the operation of specific sensors independent of the actual operation of these latter sensors. At the same time, the electronic data entering and exiting the onboard computer including the actual data associated with the network of sensors and actuators can be continuously monitored and analyzed by the external computer.