Could be a resistor problem.
Check the resistor if it only works on high Check the resistor if it only works on high Check the resistor if it only works on high
The heater motor speed resister is burned out.
Unless it is a high efficiency model 14 SEER or greater, it is only supposed to run on high speed in a/c mode and low speed in heat.
The blower motor speed resistor is burned out.
More than likely the blower motor speed resistor is burned out.
Probably your blower motor speed resister is burned out.
There is a resistor near the blower that is bad, known cause is high speed only.
That would be caused by the blower resistor.
If the one speed it works on is high speed, it is the blower motor speed resisitor that is bad.
blower resistor is bad
Normally when a heater blower motor only works on the High setting, it is caused by a defective Blower Motor Resistor Pack. The fix is to, Replace it. In very rare instances it can also be a defective blower motor speed control switch, but rarely.
Be more specific, if the fan speed only works on high then it is most cases a bad resistor board for the fan, a $10-20 part, available from an advance, murrays, or lee or similar auto parts store.