A modern Beijing Sonata car with a mileage of about 120 thousand km and a F4A42-1 four shift automatic transmission is equipped. Users reflect: when the vehicle is cold, whether it is linked to the forward gear or the reverse gear, the engine will stall. The stalls of the hot trucks disappear, but the transmission occasionally slips. After receiving the car: after trial run, it is confirmed that the fault is real. The gearshift skid gear is the 3 gear. Analyzing the cause of the failure, there is little relationship between the gear stalling and the shift actuator of the gearbox. The clutch of the torque converter should be the problem. The transmission will enter the torque converter locking state in the 3 gear, and the slip phenomenon happens in the 3 gear, so the locking clutch is most likely to be damaged. Lift the vehicle, release the automatic transmission oil, and find metal chips in the oil. Remove the automatic transmission and conduct a breakdown check. The oil poured out of the torque converter is very dirty. When the hydraulic torque converter is cut off, the locking clutch inside it is seriously worn out. Check the hydraulic valve body, there are a lot of metal chips inside, it is very troublesome to clean up, some slide valves are stuck. Thoroughly clean the heat dissipation system of the vehicle, replace the torque converter and the hydraulic valve body, install the automatic transmission, test the car, and eliminate the trouble thoroughly.