If the measured subcooling is more than 3°F lower than the recommended value, what should you do?

When the measured subcooling is significantly lower than the recommended value, which in this case is more than 3°F lower, it indicates that the refrigerant is not being properly condensed or is not adequately subcooled before entering the expansion device. This scenario often signifies that the system may be low on refrigerant.

Adding refrigerant in the vapor state can effectively increase the refrigerant pressure within the system without overwhelming the liquid line and compressor. This approach can help to ensure that there is enough refrigerant in the system to ensure proper operation and avoid compressor damage due to lack of lubrication or inadequate cooling. The vapor can mix with the remaining liquid refrigerant properly, leading to a balanced system eventually.

This choice is preferable because it can help restore adequate refrigerant levels and improve overall system performance without the risk of liquid refrigerant flooding back to the compressor, which can occur if liquid refrigerant is added directly to the suction side.