Air Temperature Control Blend Door Failure No Heat or no AC (dependes on where door sticks)
In all modern cars there is a tiny DC electric motor driven gear drive that moves a plastic door that blends heated and cooled air from the AC and the heater core, this is called (interestedly enough) the air temperature control blend door actuator. The motor that moves this door fails because the door starts sticking from warping, most common complaint is no heat but it can be no AC as well. To replace these parts (blend door and blend door actuator) you must remove the entire dash from the car. Always have this problem confirmed by a dealer or qualified repair shop as it is a very expensive repair.
Other causes of no heat in the cabin are engine thermostats that are stuck open (engine runs cold) or air bubbles in the cooling system.
Whenever this warning light or any other stays on in the dash it is indicating that the systems control module (or the system sensor in the case of lights like low oil pressure, door open indicators etc.) has detected a fault, it then disables the system (this applies to ABS,TCS, TCM, SRS and BCM faults and others) as long as the light is on. You must now have the dealer hook up a special electronic diagnostic tool and "scan" the system for the fault code that is stored for the problem(s)
Legend of Abbreviations. ABS Antilock brake system.TCS Traction control system, TCM Transmission control module, SRS Supplemental restraint system, BCM Body control module
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