The mileage is about 143 thousand km, equipped with Mercedes Benz S350 car with M272 engine and 722.9 transmission. Users reflect: during the vehicle running, the automatic transmission appears locking gear phenomenon, and only travels in 2 gears, the highest speed can reach 60km/h. At the same time, the dashboard displays the "transmission failure" prompt information and lights up the fault warning light. Fault diagnosis: the automatic transmission system is diagnosed by using the diagnostic instrument, and there are multiple fault codes. In order to confirm the authenticity of the fault code, carry out the clearance, and then test the car, when the fault is reproduced after the re diagnosis, the result only one fault information: the transmission ratio transmission ratio information is not reliable, the clutch slips. This indicates that there are problems in the hydraulic system or the mechanical transmission part of the transmission. The automatic transmission is removed and decomposed and checked. All the friction plates are not ablated. Check the piston of the clutch and brake, and find a piece of rubber piston lip on the low speed clutch. The pressure test, the piston does not move, indicates that the leakage is serious and can not transmit power, which affects the calculation of the transmission ratio of the automatic transmission control module, which leads to the lock of the automatic transmission. Replace the damaged clutch piston assembly (piston and seal ring), replace the sealing element and install the automatic transmission. After the trial run, the symptoms disappeared and the maintenance work ended. Fault summary: when automatic transmission, we must have clear thinking and check in place. It is not enough to inspect the mechanical parts only by observing the quality of the friction disc. This case illustrates the problem very well. The usual experience is that if there is leakage in the hydraulic circuit and piston seal elements, there must be ablation marks on the friction plates. But don't forget that if the lip of the piston is badly damaged, the piston can not move, and it will not cause friction in any case, and it will not be ablated. Therefore, experience alone is not enough, or a standard maintenance process is needed to ensure that the cause of the trouble is found and the trouble is eliminated.