A vehicle diagnosing system for performing a fault diagnosis of a vehicle by performing communication between a plurality of electronic control units in the vehicle and an external diagnosing apparatus and reading out fault codes recorded in the electronic control units into the external diagnosing apparatus , wherein the vehicle comprises: a plurality of warning lamps each configured to light when a fault has occurred within respective target ranges in the vehicle controlled by specific ones of the electronic control units , and demand an inspection or repair of the fault; and an idling stop controller configured to automatically execute an idling stop for stopping a drive source of the vehicle from idling when a predetermined condition is satisfied; wherein when the electronic control units detect a fault in the target ranges controlled thereby, the electronic control units record the fault codes; the fault codes include idling stop fault codes defined as inhibiting the idling stop, and non-idling stop fault codes not defined as inhibiting the idling stop; the idling stop fault codes are made up of first idling stop fault codes defined as energizing the warning lamps and second idling stop fault codes not defined as energizing the warning lamps ; for performing a fault diagnosis with respect to non-execution of the idling stop in a case where none of the warning lamps are energized, the external diagnosing apparatus extracts only the second idling stop fault codes, among the first idling stop fault codes and the second idling stop fault codes, from the electronic control units , and displays the extracted second idling stop fault codes on a display unit of the external diagnosing apparatus ; and in a case where one of the warning lamps is energized, the external diagnosing apparatus reads out the first idling stop fault codes from one of the electronic control units which controls the target range corresponding to the warning lamp energized. a plurality of warning lamps each configured to light when a fault has occurred within respective target ranges in the vehicle controlled by specific ones of the electronic control units , and demand an inspection or repair of the fault; and an idling stop controller configured to automatically execute an idling stop for stopping a drive source of the vehicle from idling when a predetermined condition is satisfied; wherein when the electronic control units detect a fault in the target ranges controlled thereby, the electronic control units record the fault codes; the fault codes include idling stop fault codes defined as inhibiting the idling stop, and non-idling stop fault codes not defined as inhibiting the idling stop; the idling stop fault codes are made up of first idling stop fault codes defined as energizing the warning lamps and second idling stop fault codes not defined as energizing the warning lamps ; for performing a fault diagnosis with respect to non-execution of the idling stop in a case where none of the warning lamps are energized, the external diagnosing apparatus extracts only the second idling stop fault codes, among the first idling stop fault codes and the second idling stop fault codes, from the electronic control units , and displays the extracted second idling stop fault codes on a display unit of the external diagnosing apparatus ; and in a case where one of the warning lamps is energized, the external diagnosing apparatus reads out the first idling stop fault codes from one of the electronic control units which controls the target range corresponding to the warning lamp energized. <EOS>