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1999 mercury sable heater

The heater will not produce heat when the temperature dial is rotated to the heat side of the guage, yet when turned to the cooling or AC side it functions, the question is…what would be the most probable cause(s)for this situation?

Choice 1 , electric blend door motor. 2, why is the door stuck ? look for debris inside, things fall down the defroster ducts. 3, vacuum motor. 4, switch.

Is the car reaching operating temperature (does it get warm in the engine?)

Add to that : 5 low coolant level. 6. clogged heater core. 7. open thermostat.
(Note that electric blend door problems are not uncommon.)

I could be the control connected to the dial too. It is just a variable resistor and it could have a dead zone due to wear.