What likely cause would result in frosting on the suction line and part of the compressor?

Frost forming on the suction line and part of the compressor is indicative of low refrigerant levels. When there is insufficient refrigerant in the system, the pressure within the suction line drops, leading to a decrease in temperature. This drop in temperature can cause moisture in the surrounding air to condense and freeze, resulting in visible frost or ice on the suction line.

Adequate refrigerant is essential for maintaining proper pressure and temperature levels within the system. When the refrigerant charge is too low, it not only affects the cooling efficiency of the system but also causes abnormal temperature drops leading to frost formation.

In contrast, excess refrigerant typically leads to increased pressure and, likely, warmer temperatures in the suction line, reducing the chances of frost. Low ambient temperatures alone may contribute to frost formation but are not the sole cause, as a well-functioning system may operate effectively even in cold conditions. A faulty expansion valve could disrupt refrigerant flow, but its immediate effect would more likely manifest in inconsistencies in cooling rather than direct frost accumulation unless it causes drastic refrigerant levels to fall.