Heater is blowing cool air
All the sudden the heater starts blowing cool air. The blower motor, fan, and
blower motor resistor are working well. The car is not over heating as the
engine temperature gauge is indicating normal, therefore, the thermostat is
properly functioning. The hoses feeding the heater core are hot. No coolant
leak or smell at all. The coolant system was completely flushed out when a
new water pump was installed back in August 2018. The AC system is
running very well. Any suggestion on what could be the cause of blowing
cool air when the heat knob is turned on. Thank you.