I recently had my hvac go out totally and had it at the dealership to fix it. In my case I had no air at all coming out. My problem was a bad blower motor. From what I learned when the blower is stuck on full blast it is a resistor problem. Unfortunately that is the bad news. Originally the dealer thought a resistor was my problem and it took a week for them to get the part in. I would bet that you are in need of a new resistor. The good news is once you get it, it is a simple operation to change out the old one for the new one. Hopefully it is under warrenty as I believe the resistor is fairly expensive.