The first thing that you need to check will be the coolant level..................
Defective heater resistor pack.
Maybe because your climate control system needs to be repaired....
The blower motor speed resistor is burned out.
try replacing the blower motor resistor
try replaceing the switch that controls the low to high for heater It's gonna be the blower motor resistor (near the blower motor).
Defective Heater Resistor Pack.
Check the low speed relay and the resistor at the motor
Most likely and bad blower motor resistor.
You most like have a bad blower motor resister.......
Heater Blower Resistor
This is normal for a car. This occurs because the engine has to work a little harder to pump up the heat, and this results in higher RPM.
from what i have read it is the blower motor resistor it goes bad and only lets the fan run on high google blower motor resistor and read on