After the hot refrigerant gas is condensed into liquid by the condenser, it gets sent to the expansion valve. The expansion valve is used to bleed off the pressure from the hot refrigerant liquid. When the pressure drops, the result is that the refrigerant cools to the point where it can now enter the evaporator and complete the process of getting cold air in your car on a hot day. If you have trouble keeping the A/C temperature cold, and there is plenty of refrigerant in the system, it may be possible that the expansion valve is damaged or faulty.