An orifice tube in a car's air conditioning system is a small, fixed restriction that is located within the liquid line of the system. Its main function is to control the flow of refrigerant entering the evaporator, which helps regulate the pressure and temperature of the refrigerant as it transitions from a high-pressure liquid to a low-pressure liquid and then to a gas within the evaporator. This controlled flow is essential for the efficient operation of the AC system, as it helps ensure proper cooling performance and prevents damage to the system components. The orifice tube works by creating a pressure drop across its small opening, which allows the refrigerant to expand and cool as it enters the evaporator coil. Over time, the orifice tube can become clogged with debris or fail due to wear and tear, causing issues with the AC system's performance. It is recommended to replace the orifice tube when servicing the AC system to maintain optimal cooling capacity and prevent potential damage.