Provided are a vehicle diagnostic system, a vehicle diagnostic method, and an external diagnostic device that are capable of readily or promptly confirming or diagnosing whether or not a malfunction related to non-execution of idle stop in a vehicle having an idle stop control system has occurred. The external diagnostic device has recorded therein a plurality of idle stop conditions for executing idle stop of an engine in a vehicle. Idle stop conditions that are not fulfilled when idle stop has not executed are displayed in a prescribed display unit, when stopping the vehicle when the external diagnostic device has been connected to the vehicle and the vehicle has been driven.