An apparatus, for performing a diagnostic operation on plural controllers which are coupled via different types of in-vehicle communication networks, may include at least one of a message generator, configured to generate a diagnostic message used for the diagnostic operation on the plural controller, and a message receiver, configured to recognize the diagnostic message, wherein the diagnostic message that may be communicated over the in-vehicle communication networks includes a header and a payload, and wherein the payload is determined to correspond to a length of communication data for a controller area network (CAN).