Every air conditioning system has an expansion valve or some other similar device that is designed to reduce the pressure of the refrigerant in the system. Even though the expansion valve is a simple device without any moving parts, it is the key to the air conditioning system in your car.
The air conditioning system in your vehicle is split into two halves, a high-pressure side, and a low-pressure side. The high-pressure side starts at the compressor and ends at the expansion valve and the low-pressure side starts at the expansion valve and ends at the compressor again. These two devices have similar, but opposite tasks of either increasing or decreasing the pressure. The real question is, why would you want to increase the refrigerant pressure in your air conditioning system if you were going to allow it to drop back to a low pressure just a few moments later?
The reason for this seemingly pointless path for the refrigerant in your system is to take advantage of the cooling effect you get when allowing a high-pressure fluid to quickly decompress. You may have experienced this when using a can of spray paint. If you empty the can quickly you will notice the can becoming cold in your hand. The expansion valve in your vehicle creates the same effect by allowing the refrigerant to decompress quickly. This causes the refrigerant to be cooled down near 32 degrees in your car! This cold refrigerant is then sent to the evaporator in your car where it can cool the air in your cabin.
The expansion valve is critical to the operation of your air conditioning so if it is malfunctioning, it can easily render your air conditioning system inoperable. The expansion valve is a usually just a very small opening with an expansion void after the opening so really the only way the expansion valve can malfunction is if it becomes clogged.
During normal use, there isn’t any reason that the expansion valve should get clogged. Your air conditioning system is a closed system so it is normally impossible for any debris to enter the system. One way the expansion valve could get clogged is if there was a leak in the system where it was open to the atmosphere and contamination entered the system that way. Another way the expansion valve could get clogged is from wear products from your compressor if it is starting to fail. Lastly, an expansion valve could get clogged if low-quality refrigerants are used or low-quality leak stop products.
Some leak stop products use tiny particulates to literally clog up leak spaces in your air conditioning system to help keep refrigerant contained. These leak stop products can often clog your expansion valve as well rendering your system inoperable. You can prove you’ve got a clogged expansion valve by measuring the pressures around your system and checking for an abnormally high pressure on the high-pressure side of the system along with no cooling effect.