More likely than not, the Blower Motor Resistor Pack is defective and needs replacing.
Replace heater motor resistor.
vacuum leak
Maybe because your climate control system needs to be repaired....
If the engine is at the proper temperature and the coolant is full, you have a restricted heater core.
Most likely a failed blower resistor.
If it has only high speed, the blower motor resistor has to be replaced to get all speeds back.
blower motor resistor is burned out
The blower motor resistor pack is bad.
Blower fan inoperative or only works on high speed.
Check the resistor pack on the evaporator case under the hood
Most likely the blower resistor has failed. It is located under the hood in the side of the heater box.
I had the same problem - it's a bad resistor. Most mecahnics can change this and it didn't cost a lot if I remember.