ac runs and so does heat at same time no ac in car

Asked by Jul 24, 2016 at 11:19 AM about the 1998 Mercury Grand Marquis 4 Dr GS Sedan

Question type: Maintenance & Repair

ac runs and heater will not shut off

1 Answer

18,905

The heater core never shuts off. Even on the blazing hottest day it sits there in your dash, a little radiator dispensing heat. All that happens when you switch the temperature knob from heat to cold a little plastic door (called a blend door) swings over to direct air flow from the blower motor to go over the AC evap coil instead of the heater core. If you have good AC and COLD air (check the compressor is running) and HOT at the same time, it's a sign your blend door is stuck open to the heater core. Usually it's the blend door actuator that failed. The part costs about $35. The labor, however, to get to the heater cores plenum box is expensive. You either have to remove or loosen the dash, pull radio, passenger air bag, drop glove box, to gain access to it.

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