heater blowing out cold air when switched to heat,what could be the problem?
They also have a vacuum operated valve that shuts the hot coolant from going through the heater core, its so that when you have the ac on its not fighting against the hot heater core. Sometimes these valves get stuck. I just remove it when that happens and I don't notice any difference with the ac.
Sounds like either a faulty/broken blend door or bad actuator motor on the HVAC unit.
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