Probably the fan switch - recall out on many of them. Some (like mine did) melt internally. Costs about 12 bucks and is easy to install. You will nose when the resistor block (separate from the switch) goes. The fan switch will cause the whole HVAC system to act up. I didn't want my cars to sit at the dealer all week waiting for the recall work while I drove a $25 a day rental car.
