Common transmission problems include low fluid levels and problems with the torque converter, solenoid or clutch. These issues occur with automatic transmissions, which are the most common in passenger vehicles as of 2015.
Automatic transmissions require fluid to operate correctly. Fluid often leaks through worn seals in the transmission or driveshaft. Sometimes leaks allow radiator coolant to leak into the transmission fluid, causing cross contamination. Transmission fluid problems cause the gears to slip, or the vehicle to shift erratically or slowly.
Worn needle bearings in the torque converter cause the transmission to make strange noises when operating in the driving gears. The noises stop when operating in neutral. Without replacing the worn or damaged bearings, transmission failure is likely.
Automatic transmissions use a solenoid valve to control the flow of fluid through the system. Operating the vehicle when the transmission fluid is low damages the valve. When there are electrical problems, it does not work correctly. Vehicles that do not shift properly, even when the fluid level is full, are likely experiencing solenoid valve problems.
Automatic transmissions use a fluid clutch in the torque converter. It sometimes jams and locks the solenoid valve. The problem initially resembles a low fluid problem, but eventually there is a loss of power.