Restriction in the System
During the initial inspection, the inlet and outlet lines of the receiver drier should feel warm and about the same temperature. If there's a difference or frost on one of the lines, there's a restriction in the drier.
The inlet line contains liquid refrigerant coming in from the condenser, and the outlet line contains cleaned and dried liquid refrigerant leading to the expansion valve. The receiver drier's function is to clean and dry the refrigerant as it flows through its filter and desiccant bag.
The receiver drier absorbs moisture from the refrigerant and acts as a reserve liquid refrigerant tank for the expansion valve. If debris in the system accumulates in the drier, a restriction may form, causing a difference in line temperatures. Refrigerant temperatures change with system pressures. In an air conditioning system, if a restriction reduces pressure and flow, the temperature will drop, and frost appears at that spot.