In a vehicle diagnostic method and an external diagnostic device, sensor detection values acquired by communicating with an ECU are measured to determine whether the values are in a normal range while a vehicle is idling. After an engine is started, an operation to measure soundness is prohibited at least until the sensor detection values including the engine rotational frequency reach a threshold value region indicating the completion of the warm-up operation. The fluctuation state of the sensor detection values is displayed on an operation screen until the sensor detection values reach the threshold value region.