My mechanic tells me I need a new a/c compressor and in addition to that the receiver drier and the expansion valve. Why do these parts have to be changed if it’s just the compressor?”
OK, here’s why. If a compressor goes bad, it will often throw crud and possibly metal through the system. The expansion valve can collect this debris and then make the system not work after you put a new compressor on. The receiver drier also can collect this debris, AND anyway, EVERY single time an AC system is opened up, the receiver drier should be replaced because one of its duty is to dry the system of moisture. Once the system is opened up to replace a part, the drier collects all the moisture and is ruined. So, it should be replaced when even an AC hose is removed and replaced. Hope that explains it for you.