Hello and Thank you for choosing Just Answer.
Before I can assist you, I will need to know:
Manual, I guess....There is not like a temp guage that automatically turns on the air.
Well to put it easier:
Yes, The knob to the far right controls the setting, the middle one controls the temp and the left one controls the speed. I replaced the one that controls the speed.
Ok Thanks. The problem you are having with the fan stuck on high is due to a bad Blower Motor Resistor. This resistor is about $40 and is located under the dash on the right side. You simply unscrew it and unplug the connector and replace with a new one.
Here is the removal procedure: