A 2001 milled TOYOTA cruiser with a mileage of over 200 thousand km, 5, automatic transmission and four wheel drive. User response: the car has abnormal sound when driving in 1~3 and 5 gears. After receiving the car: road test, to determine the abnormal sound is emitted from the automatic transmission. The transmission was removed for inspection without obvious signs of oil leakage. Hanging the gear lever into the forward gear and rotating the input shaft with your hands, you can hear the mechanical noise inside the transmission. There is irregular seal seal on the shell joint of the transmission, which indicates that it has been dismantled and repaired, so there is possibility of human failure. The automatic transmission was disassembled and checked. It was found that there was a large amount of gear oil in the connection part between the transmission and the distributor. These gear oils leak out of the transmission, while the transmission housing is not damaged. Therefore, the only leakage point is the output shaft oil seal of the transmission. The inner diameter of the oil seal and the outer diameter of the input shaft of the distributor are measured. The former is about 3mm larger than the latter. Continuing to decompose the transmission, it is found that the meshing surfaces of a large number of gears are severely worn, and the clearance between most of the bearings is extremely poor, and the transmission has no repair value. Change the transmission assembly, the fault is completely eliminated. The fault is caused by the incorrect operation of the maintenance personnel during the replacement of the oil seal. The part number of the faulty oil seal is 90311-43007, and the part number of the standard oil seal is 90311-40007. The difference of one digit results in the scrapping of the transmission assembly, which calls for enough attention. In fact, when the oil seal is installed in the output shaft of the transmission, it is impossible to accurately judge the combination of the oil seal and the shaft through the visual inspection, and the transmission and the distributor can not be directly inspected after assembly. Therefore, the size of the oil seal must be measured according to the operation specifications and the actual measurement size shall prevail so as to avoid similar accidents.