An A/C expansion valve is a critical component in your air conditioning system. It's a metering device because it controls the flow of refrigerant into the evaporator. Within the air conditioning system, this device receives high-pressure, low heat liquid-state refrigerant from the condenser and lets in to the evaporator only a small amount of liquid so there's space for the liquid to convert back into a gaseous state. The expansion valve has the ability to sense the temperature of the incoming liquid and open or close incrementally to allow the correct amount of liquid into the evaporator. Symptoms of a failed expansion valve usually include air temperatures coming from the vents that are inconsistent with the temperature settings of the air conditioning controls, or one continuous temperature regardless of the control settings. These occur with the temperature-sensing needle and bulb stop working, or the spring that holds the valve closed loses its tension, causing the valve to stay open more than it should. If you like having cold air at a moment's notice, it's best to have the expansion valve checked and replaced as soon as possible. Not replacing a faulty expansion valve could result in damage to the other components in the A/C system as well.
Although an air conditioning system can be statically regulated, it's not nearly as efficient as having an expansion valve performing the regulating. Not only is having the valve more efficient, it also makes the A/C system more accurate, based on the temperature setting the user chooses. The expansion valve has existed in one form or another since the mid-1800s, and it was first used with steam engines, most famously for locomotives. An expansion valve isn't unique to transportation either. There's one used on basically any liquid-gas changing device such as your home A/C system or your refrigerator.
An expansion valve can be changed out by the average do-it-yourselfer, however we always recommend having the work done by a certified mechanic who is licensed to handle and dispose of refrigerant. Below are a few brief steps to take when replacing the expansion valve: