The present invention provides a vehicle diagnostic system comprising an electronic control unit for controlling and monitoring vehicle equipment, a radio communication means for transmitting a vehicle diagnostic information to a telecommunication equipment located outside the vehicle or for communicating between the vehicle and the telecommunication equipment located outside the vehicle and for transmitting a communication diagnostic signal to the electronic control unit and receiving a response signal to the communication diagnostic signal, and a communication line for connecting the electronic control unit and the radio communication means wherein the response signal is utilized to determine if the communication line is properly functional.