A fault diagnosis method for a vehicle includes steps of: detecting, by a controller, whether a reference engineering code (REC) has been generated; if the reference engineering code (REC) has been generated as a result of performing the detecting step, measuring, by the controller, a time for which the reference engineering code (REC) has been generated; after the measuring step, calculating, by the controller, a percentage of the reference engineering code (REC), based on both the time for which the reference engineering code (REC) has been generated and a preset time; after the calculating step, storing, by the controller, the reference engineering code (REC) and the percentage of the REC; and after the storing step, when a request signal is received from a diagnostic device, providing, by the controller, an output signal that includes the reference engineering code (REC) and the percentage of the REC to the diagnostic device.