Delayed Shifting

• If you shift from park into drive but the vehicle does not engage for several moments and then finally shifts into gear, there is likely a transmission problem. If your transmission fluid is low, you may have a leak in the pressurized system that has caused the delay. If your fluid is full and not leaking, then there is a more serious problem with the transmission.
  
  

Noises

• When your car shifts gears, pay attention to any unusual noises that occur. If you hear any deep grinding, clunking or squealing as your car shifts, especially if it appears to struggle as it does so, then you have a transmission problem. Try to hear where the noise is coming from. If it is coming from under your feet, then the transmission itself is likely the problem, and a replacement might be required.
  
  

Won't Go Into Gear

• If a car won't go into a given gear, the transmission might need to be completely replaced. Again, this problem could be caused by a leak that de-pressurizes the system. Barring that problem, however, if a vehicle will not enter a given gear at all, then there is serious damage somewhere within the system. Due to the complexity of an automatic transmission, most mechanics will prefer to replace the transmission entirely.
  
  

Feel

• Paying attention to how the shifting feels is as important as listening for noises and watching your gauges. If shifts are no longer smooth, especially if you notice them getting rougher over time, then the gears or synchronizers inside your transmission could be wearing out. If you can feel the gears grinding when shifting manually or if you feel the vehicle shake or stutter when an automatic transmission shifts, you have reason to suspect this problem.