An automatic transmission flush is a primary component of preventative maintenance with an automatic transmission. As transmission fluid ages, it picks up microscopic particles of friction material and metal that can eventually clog the tiny valves and orifices in the valve body. If the transmission isn’t flushed regularly, these particles can build to a point where the transmission filter becomes clogged. The transmission will stop working because the fluid is unable to flow. Generally speaking, this maintenance procedure should be performed whenever your transmission fluid turns a dirty brown color or smells burned. This is a very distinctive scent that can be mistaken. Manufacturers say the fluid flush should occur at 50,000 to 75,000 mile intervals.
For more info, see Automatic Cars: Which Make and Model Fits Your Needs?