The AC evaporator cools the air before it is blown into the cabin. It is comprised of small round or oval tubes. The cooling agent is first compressed by the compressor and then decompressed by the evaporator. As a result of wear, the evaporator becomes clogged or may corrode and leak. Serious leaks may even cause a seizure of the compressor. This is why the evaporator should be changed as soon as its failure is discovered.