To diagnose automatic transmission problems, check for a low transmission fluid level, which leads to poor shifting of a vehicle and gradually causes damage to the transmission, and check for leaks in the filler tube base, the drain hole below the transmission, and the area between the engine and transmission. Also check for a clogged filter, and replace the filter if necessary.
When searching for leaks, also look at the selector shaft that connects the gear shift to the transmission. Another place to check is the speed sensor mounting point, which refers to the electronic sensor or cable attached to the transmission housing. In some cases, vehicles with transmission coolers have leaks that cause the transmission fluid to ooze into the radiator fluid.
If you cannot fix your car's automatic transmission problem, consult an experienced vehicle repair technician, and inform him about your findings when diagnosing the problem. Tell the technician about the presence of any leak and its severity.
An automatic transmission may need to be rebuilt if it's significantly worn or damaged. However, this should be the last option, as disassembling a transmission is costly. Repair shops usually offer a warranty for rebuilding a transmission. Keep your newly rebuilt transmission running properly for a long period by performing regular maintenance.