In a vehicle diagnostic system and a vehicle diagnostic method, malfunction codes recorded in each ECU in a vehicle include idle stop malfunction codes that disallow idle stop, and a non-idle stop malfunction code that does not disallow idle stop. The idle stop malfunction codes include: a first idle stop malfunction code that operates a warning light provided corresponding to a specific ECU; and a second idle stop malfunction code that does not operate the relevant warning light. An external diagnostic device extracts, from the plurality of ECUs, only the second idle stop malfunction codes, among the first idle stop malfunction codes and the second idle stop malfunction codes, when performing malfunction diagnosis for non-execution of idle stop.