Manufacturers typically suggest that automatic transmission fluid should be changed every 20,000 to 25,000 miles. This is often done at service stations, but some people do the job themselves.
A car's transmission helps send power from the engine to the wheels. Automatic transmission fluid lubricates the gears and torque converters that are involved in this process. When the pressure changes in the fluid, the transmission changes gears. As the car drives, the temperature of the fluid rises to 175 degrees Fahrenheit and higher. When temperatures are too high for too long, the fluid starts to break down, and, eventually, it needs to be replaced.