The dishwasher drain impeller is responsible for forcing water out of the appliance and through the drain hose. Typically made of plastic and shaped like a propeller, the impeller is powered by the dishwasher's motor. As the motor turns the impeller, it creates suction that pulls water through the drainage system and expels it from the appliance. Over time, the impeller can become damaged or worn out, leading to slow drainage or complete failure to drain, resulting in a buildup of dirty water. Signs of dishwasher impeller issues include unusual noises or slow drainage. Replacing the impeller is a relatively simple task that can be handled by a professional.