Vehicular electronic control system and electronic control unit, program, and storing member for the same

A vehicular electronic control system (10) includes multiple electronic control units (ECUs) (1, 2, 3, 4) to which a tester (50) is connectable for diagnosis . Each ECU (1, 2, 3, 4) stores a vehicle identifying number (VIN) code that is transmitted to the tester (50) for providing various kinds of information on a vehicle (100). A priority level is assign to each ECU (1, 2, 3, 4) so that the ECUs (1, 2, 3, 4) transmit the VIN code in order of precedence when the ECU (1, 2, 3, 4) having higher priority is unable to transmit the VIN code when another ECU (1, 2, 3, 4) has not transmitted the VIN code. Moreover, each ECU (1, 2, 3, 4) monitors a communication line (L) when received a VIN code transmission request from the tester (50) and detects an output indicative the VIN code sent from another ECU (1, 2, 3, 4) to the tester (50). When the ECU (1, 2, 3,4) has detected the output, it terminates the VIN code transmission and do not transmits the VIN code to the tester.