We have a 16 year old York heat pump that ices up and does not completely defrost. Just the bottom third of the exterior will defrost and then it exits defrost mode and returns to normal operation. We just had the defrost sensor and circuit board replaced 2 weeks ago. Before doing so, the HP was a block of ice with no defrosting at all. Weather has warmed up here- 52 degrees high with 47 as low temperatures. Icing was much worse when it was much colder out.