The Nissens AC Expansion Valve - 999228 is a component of a vehicle's air conditioning system. It is responsible for regulating the flow of refrigerant into the evaporator, which cools the air that is circulated into the cabin. The expansion valve works by controlling the pressure and temperature of the refrigerant, ensuring that the air conditioning system operates efficiently. It is designed to fit specific makes and models of vehicles and is a crucial part of maintaining a comfortable and functional air conditioning system.