The Nissens AC Expansion Valve - 999378 is a component of an air conditioning system that regulates the flow of refrigerant into the evaporator. It is designed to control the temperature and pressure of the refrigerant, allowing for efficient cooling of the vehicle's interior. This expansion valve is made by Nissens, a reputable manufacturer of automotive parts, and is compatible with various makes and models of vehicles. It is a crucial part of the AC system and is responsible for maintaining the proper functioning of the air conditioning unit.