I just had same problem...was told by autodealer to turn on heat and let cold temp setting cycle through...then shutt off and star up again.shud fix....so i defrosted for 5 mins at high heat.then synced heat to low woith high fan for another 5 mins.shut off vehicle.started vehicle and slowly increased synced heat...IT WORKED!
I replaced the Blend door actuator to na avail. It was a plugged Heater core. A big job requireing the dealer. 10 hours of labor to remove the dash and replace the core. $1800. Never heard of a core cgoing bad in 6 years.