Common symptoms of automatic transmission problems include gear slippage, transmission fluid leakage or transmission fluid that is dark and smells burnt. In addition, rough or clunky gear changes can also be a sign of transmission trouble.
In a properly functioning automatic transmission, gear changes are quick and smooth. When the driver presses the accelerator, engine power should be delivered in a constant and predictable way. If there is hesitation, or the car seems to be lurching, a qualified mechanic should be seen immediately.
Inspect the automatic transmission fluid. Transmissions don't consume oil like engines do, so if the level is low, there may be a leak. The fluid should be reddish, clear or smell slightly sweet. Transmission fluid that is dark or smells burnt is a serious problem that requires urgent attention by a mechanic.
There are serious problems that have less obvious warning signs. Any kind of high-pitched whining noise, grinding or buzzing sound while driving may mean the transmission is at risk of being damaged. Even if these noises are infrequent, neglecting them can allow them to get worse.
In modern cars, automatic transmissions are heavily controlled by the car's onboard computer, and irregular or unusual transmission behavior may be due to an electrical glitch. If the car's check-engine light is illuminated in the dashboard and transmission issues arise, a mechanic who has a computer diagnostic tool can provide more information.