Abstract: A vehicle diagnosis system can handle with OBD III as well as OBD II by only adding a transponder to an OBD II system. The vehicle diagnosis system includes an engine ECU for controlling an engine device such as injector and storing a diagnosis data, a K-line connected to the engine ECU, a transponder connected to the K-line, and a connector connected to the K-line for detachably connecting an external scan tool that reads the diagnosis data stored in the engine ECU. The transponder receives a request from a receiver system by radio, reads the diagnosis based on the request, and sends the read diagnosis data to the receiver system.

Abstract: A vehicle is equipped with an engine ECU as a vehicle control unit and a transponder unit, which are connected through a communication line. The engine ECU sends out a response data such as a failure self-diagnosis information of the vehicle to the transponder unit through the communication line in response to a request transmitted through a radio wave communication from an external control station. The transponder unit transmits the response data through the radio wave communication to the control station in return to the request. The engine ECU has a circuit which receives the response data through radio wave communication. The engine ECU compares the received response data with the original response data sent out through the communication line. If the two response data agree, it is determined that the response data is transmitted to the control station successfully. However, if the two response data differ from each other, it is determined that there occurred a transmission failure.