The receiver dryer is a component within the A/C system that pulls moisture from the refrigerant, the liquid/gas that helps cool down your interior. It contains a small desiccant bag inside that collects the moisture, and if the A/C system is left open the desiccant will collect moisture from the outside air and render the receiver dryer useless, as the desiccant bag will have too much moisture already collected. Replace yours to ensure your A/C system is running efficiently to keep your interior cooled down.