The air filter keeps dirt, dust, and other airborne particles from getting into your home. A common cause for window air conditioners to not blow air is a restricted air filter. If the air filter becomes dirty or clogged, there will be little or no air flow over the evaporator coils. This causes them to become too cold and frost or ice can start to build up, resulting in no air being circulated into the room. The air filter is positioned in front of the evaporator which is located behind the front grille. Regular replacement or cleaning of the air filter is essential to maintaining the efficiency of the air conditioner. This is the right-side air filter for room air conditioners, and is sold separately from the left-side filter.