Automatic transmission is probably the most complicated part of a car. Only a technician can fully understand automatic transmission because of its complexity. But having to diagnose the transmission for problems do not require technical background. All you have to understand is the symptoms for the transmission problem. One of the problems that often encounter is the automatic gear shift problem of the automatic transmission. The causes of this problem is plenty and the most worst of all is the damage transmission, however the most easy to fix is cause for the problem is an insufficient amount of transmission fluid since this can also cause the gear shift not to work properly.
Also, having a bad fluid pump and a problem on gear selector linkage can also make the gear shift not working properly. Fluid pump is the device that pumps hydraulic fluid inside the transmission to make the gear inside it lubricated and works properly making also the transmission to work. As for the gear selector linkage this is the device that identifies what gear will be operated, therefore when the linkage is out of adjustments it will not identify which gear is going to be operated and this will make the gear shift not to work.
When you are attempting to do a automatic transmission troubleshooting, the first thing to check is the amount of transmission fluid, if the fluid is not sufficient enough just add enough amount of fluid to fix the problem, nevertheless the transmission should be inspected to determine why the fluid inside the transmission loss some fluid, but if the fluid is sufficient enough after checking then the transmission is needed to be open to determine other cause of the problem. Repair of transmission problem should be done by a professional due to complexity of this part. Bring your car to a reputable car shop for checking and to determine the real cause of the problem.