I recently went through the same issue. The center button of the switch was no good even though the light came on showing the AC was on. I wired in a toggle switch temporarily until I found a control unit on ebay.
Once I had the old control unit out, I took it apart and found evidence of high temps and arcing within the switch. However, the fan speed were all still working. I guess I was lucky that the old control unit I bought was still good.