What typically causes an expansion valve to frost?

The presence of frost on an expansion valve is often indicative of a refrigerant issue, specifically related to the flow of refrigerant. In this case, when a valve is leaking, it allows refrigerant to escape in unintended ways. This can lead to lower pressure and temperatures in the area around the valve, and consequently, moisture in the air surrounding the valve can freeze and form frost.

A leaking expansion valve can disrupt the normal operation of the refrigeration cycle, contributing to an improper balance of refrigerant in the system. This disruption can lead to overly low temperatures at the point where the refrigerant exits the valve, causing the moisture in the air to condense and freeze.

In contrast, high refrigerant pressure is typically associated with overheating or improper cooling conditions, not frost. Low refrigerant levels could potentially lead to insufficient cooling, but it generally wouldn't cause the valve itself to frost. A clogged filter drier might restrict refrigerant flow and potentially disrupt the system, but it is not a direct cause of frost on the expansion valve itself in the way a leak would be. Thus, a leaking valve is the most fitting explanation for the frosting phenomenon.