Frost formation on the suction line of an air conditioner near the service valve is a sign of malfunction in the refrigeration system and is usually caused by either low or excessive refrigerant charge. If the amount of refrigerant is below the standard level, the pressure and temperature within the evaporator drop. As a result, the refrigerant returns to the compressor at a lower temperature, causing moisture in the surrounding air to condense and freeze on the suction line.

On the other hand, if the refrigerant is overcharged, liquid refrigerant may enter the suction line instead of vapor. This can significantly reduce the temperature of the line and also lead to frost buildup. In both cases, frost on the suction line indicates improper system operation and serves as a warning to prevent potential compressor damage.

To resolve this issue, system pressure must be checked using a manifold gauge, and the refrigerant charge must be accurately adjusted. Only an experienced technician can determine the correct amount of refrigerant and restore the air conditioner’s proper performance.