What could be the most probable cause of an iced suction line in an air conditioning unit with low ambient temperatures?

An iced suction line in an air conditioning unit operating in low ambient temperatures is often indicative of issues related to refrigerant charge. When the system has a low refrigerant charge, it can lead to a dramatic drop in pressure and temperature within the evaporator coil. This drop can cause the moisture in the air surrounding the coil to freeze on the surface, forming ice on the suction line as well.

In low ambient temperatures, the conditions are such that the air surrounding the evaporator is already cooler, increasing the likelihood of ice formation. Under these circumstances, a low refrigerant charge exacerbates the cooling effect, intensifying the freezing process on the suction line. Proper operation of the system requires a sufficient refrigerant charge to maintain desired pressure levels and effectively absorb heat.

Other options like a dirty evaporator coil, plugged air filter, or a bad indoor blower relay can contribute to system inefficiencies and temperature issues but would generally lead to reduced airflow and performance rather than specifically causing the refrigeration line to ice over, especially in the context of low ambient temperatures.