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If proper coolant flow through the cooling system is verified, and heater outlet air temperature is still low, a mechanical problem may exist.
MECHANICAL PROBLEMS Possible locations or causes of insufficient heat:
If the heater outlet air temperature cannot be adjusted with the temperature control knob on the heater-A/C control panel, the following could require service:
The mode doors are vacumm controlled, but the blend air door(temp) is electrically operated. You may have a bad motor or broken door. If you remove the motor and try to operate the door while the vehicle is running and see if there is any change.