What would cause the fan to work only on the highest setting?

pull the knob off you temp switch and put it on the blower control and see if it works..i had the same problem and it was a bad knob Possible switch, resistor, relay or blower motor. Possible heater blower motor resistor. Most manufacturers let the fan override the resistor when high is selected for safety reasons Most often manufacturers use a resistor block containing bare wound wire resistors to vary the fan speed. The high speed settings simply puts the full 12 volts to the fan motor (no resistor involved). The problem is that - for whatever reason - they also include a thermal protection device that is in line with the resistors that are used to reduce fan speed. These little diode looking devices are located usually in the middle of the resistor group on the resistor block. When this thermal sensor fails, either thru overheating, or age, or design, then the current can't flow through the resistors to provide lower fan speeds - only the high speed position functions. In my personal humble opinion, I suspect that these thermal protection devices are designed to not to last too long!! My assumption is that their purpose is to prevent the resistors from overheating if for some reason the air flow thru the air duct is blocked (leaves, snow, what ever). I've had this happen on newer Fords and Chrysler vans. So now when there is a failure I clip the thermal device out of the block and solder in a small piece of thin copper wire which restores all fan functions. Is this practice dangerous. Yes, IF you allow flamable material to build up in the air duct (not sure how this could happen - but anything is possible).