Automatic transmissions/transaxles use a filter on the inlet side of the transmission’s hydraulic pump. Different types of filtering media may be used including a fine mesh screen, paper, or felt for filtering media.
A transmission filter prevents harmful contaminants from entering the hydraulic system, where they can increase wear and cause scoring and sticking of hydraulic control valves. Additionally, if a major part fails inside the transmission, the filter may prevent pieces of that part from contributing to a more catastrophic transmission failure. Normally transmission filters trap metal chips from hard parts like gears and bushings and the normal fine material that results from wear of the hydraulic clutch facings and bands.
Your car’s automatic transmission filter and fluid should be changed periodically according to the schedule in your owner’s manual. Although some maintenance schedules may claim that the transmission fluid or filter doesn’t need to be changed for the life of the car, remember that the average driving situation falls into the "severe" maintenance category due to short trips and stop-and-go driving. Some shops offer transmission flushing and filling, which is intended to remove more contaminants than simple draining of the transmission. If you decide to have this service performed, make sure that the transmission pan will be removed in order to change the filter before refilling it with new fluid. If the filter is not replaced, contaminants from the old fluid, along with those dislodged during the flushing process, could impair flow through the filter and lead to transmission problems.