The most common “fix” applied to many systems is to add refrigerant. If a system is operating with low pressures or freezing up, many techs will add some refrigerant. Homeowners often actually ask for techs to add refrigerant, thinking that more refrigerant must mean colder air. However, adding refrigerant may not actually fix the problem. In fact, often it may make things worse.
For example, if a system has low airflow it will have low pressures, and often will freeze up. The reduced load will cause low superheat and refrigerant floodback. Adding refrigerant just makes the floodback worse, shortening the compressor life. An undercharged system would have a high superheat. You should always check the system airflow, superheat, and subcooling before adding refrigerant.
Another common example is a system with a refrigerant restriction, such as a plugged up filter drier. Again, both pressures will be low, and BOTH the superheat and subcooling will be high. This can look similar to an undercharge, except for the subcooling. An undercharged system will have a low subcooling. Adding refrigerant fills up the condenser, raising both the high side pressure and the already high subcooling. It may marginally improve the low side pressure and capacity. However, it forces the system to run at an excessive compression ratio and uses lots of power trying to force the refrigerant through the restriction. A far better solution is to remove the restriction.
Finally, a clogged or stuck expansion valve behaves like a refrigerant restriction. Failed expansion valves used to be quite rare. Unfortunately, they are pretty common today. Between the valves that were fouled up because of the compressor manufacturing problem and the valves that become clogged with black copper oxides, failed expansion valves have become all too common. The symptoms are identical to a refrigerant restriction: low pressures, high superheat, and normal to high subcooling. If someone has already tried to “fix” the problem by adding refrigerant, then the high side pressure may be high and the subcooling will be very high.
Please don’t make the problem worse. Before adding refrigerant to a system with low pressures, first check: airflow, superheat, and subcooling. A truly undercharged system will have adequate airflow, a high superheat, and a low subcooling. And if the system is undercharged, then maybe you should try to figure out why.