Abstract: A diagnosis system conducts diagnosis with data read out of an electronic control unit with a communications protocol installed in a vehicle. The system has an integrated circuit to be an communications interface suitable for the protocol. The circuit can be made suitable for any communications protocol by reprogramming logical functions of the circuit with design information. Instead of such circuit, the system may have a main processor and a subprocessor. On one hand, the main processor conducts virtual data communication with the control unit for diagnosis. On the other hand, the subprocessor executes a communications processing program suitable for the protocol to have data communication with the control unit. The subprocessor receives data from the main processor and transfers the data to the control unit and vice versa for the virtual data communication.