I have a 2017 Sonata. The AC works and the heat too. But lately the turning on the AC I heard a loud flap and then just hot air comes out. Sometimes if you turn dial on fan all the way to both sides or turn the dial to blue to red (temp) they back and forth you can hear the flap sound again and then the AC cold air comes out of the vents. Now the flap sounded again and nothing but hot air comes out. I am at the Hyundai dealer and they look at me like they don't believe me. They told me for $220, they would do a ac diagnostic.

Could it be theTemperature control motor actuator?

 

Yes !

 

Did not have to pay anything. They said the compressor was bad and is under warranty. They will install next week.