PURPOSE: To discriminate the type of an on-vehicle electronic control unit(ECU) without providing a terminal for activating a special mode therein by outputting a discrimination code showing the type of ECU from a warning terminal through a test terminal grounded, and, then, detecting the code.

CONSTITUTION: The test mode terminal TS of ECU 3a is deleted and the test terminal TC thereof is grounded, so that diagnostic code output from the warning terminal W of the ECU 3a can contain an discrimination code showing whether the output is for a two-wheel drive vehicle or for a four-wheel vessel. At the time of the shipment of a completed vehicle from a factory, the diagnostic central connector 1 is connected to a service tool, and the test terminal T1 of the connector 1 is shortcircuited to an earth terminal G for grounding the test terminal TC of the ECU 3a. As the result, the diagnostic code of the prescribed pattern is outputted from the terminal W of the ECU 3a, and a meter lamp 6 flashes. According to this construction, the type of an electronic control unit can be easily discriminated, and inspection efficiency at the time of the shipment of a vehicle from a factory can be improved.