My '13 Sonata with 50K miles, yes I drive a lot, 85% of it is highway mileage, all of a sudden today on the way home I felt it getting hot in the car. The a/c was on the and fan was on low so I put it all the way up on high and though it sounded right it resulted in only a little more air coming out. I experimented with fan speeds, vent settings, a/c on and off etc. It just sounds like there’s a door closed somewhere. With it on full blast i was able to get some air leaking out enough to keep me from opening the windows but what’s up? The temp gauge never changed the whole time. When I got home I tried cycling the car on and off a few times but nada. Back in the day I would’ve dug around under the dash to see if there was a disconnected vacuum line or something like that but I’m sure it’s much more complicated than that now.
Does the car have a cabin air filter that may be clogged?
Check with dealer, it may still be covered by warranty.
This should still be under warranty. If it is, don’t try to fix it yourself or bring it any other shop. Doing so may void the warranty for any other A/C problems.
There are probably computer actuated vent doors getting stuck in the wrong position. Good comments above, since it is under warranty presumably, let the dealer handle it. If it is not under warranty, getting the diagnostic codes read is probably the next step. No good? I’ve heard of people disconnecting the battery in situations like this. Sometimes – unsure if this is the case w/your car, but it applies w/some cars – the door actuator motors get confused, and fail to understand what position they are in. When the computer is reset by disconnecting the battery, sometimes they will re-initialize and work, at least for a while. The last idea has some problems associated with it, anytime you disconnect the battery the ECM will lose information it may actually need for the car to start and run correctly.
thanks for the comments. I have a dealer app’t next week so we’ll see!
Please report back next week. That way we all learn together.
Now they work?! I garaged it for a week and used my wife’s car for work. Got in it last night, turned key on and the vents were open and working. Started/stopped the car several times they still work. Took it to the store and still works. Must be a switch thing or maybe even a computer thing. Anyhow if it keeps working I’m canceling the dealer appointment and we’ll see what happens. thanks all.
Almost a month later and it’s still working. Go figya!
So since my last report on 6/14 it’s worked perfectly 'till the other day when it did the same thing on the way home from a long trip. Luckily it wasn’t all that hot out so it was tolerable. On the way back I stopped twice but it kept it’s grip on the flappers. But the next morning after being in the garage overnight, back to normal. I’m convinced it’s a computer glitch but it’s not enough of a problem to take it to the dealer to fix, maybe to sell though. It’s been a great car otherwise though. Just replaced the front pads and rotors a few weeks ago at 119K, and they still had material left on them! Installed ceramic pads paired with slotted rotors that I bought from Pep Boys and couldn’t be happier. Much better stopping distance, no brake dust and no squealing.
You’re going to have a hard time selling a car with incorrect or poor airflow . . . I would lowball you big time, if I was even considering the car, and so would many other guys
I hate to say it, but you should have had it fixed for free, while the car still had warranty
Glad to hear your brake job went well 
I wonder if the evaporator is freezing up.
I think you’got a vent door that is sticking or else it is not being commanded to open by the computer. I imagine they are electrical and not vacuum but don’t know. I really think its going to have to go to the dealer while it is doing it so that the can hook up their computer and see if the door is being commanded to open or not. Would it still be under warranty though at 50K? If so they might give you a break since it developed while it was under warranty.
I did have my Buick go to default while sitting at a train crossing once but that was a vacuum issue and default was to the defroster. The hose had dropped down on a hot spot on the engine and developed a leak in the hose. Easy to find and easy to fix but it did get a little warm with the cold air only going to the windshield.
“just replaced the front pads and rotors a few weeks ago at 119K,”
Way out of warranty
I dunno, the air that is coming out is cold, it’s the airflow itself that is way down.
Is cold air coming out anywhere else in significant amounts?
Gotta be the doors or the friendly rodents that love to get in the cabin comfort section ,hope you get er fixed ,stuff like that ,while not critical is annoying .
Traded the car in, problem solved! Actually it never occurred again after my post. Go figure
Ice build-up in the evaporator, the compressor should cycle off/on to keep the evaporator temperature above freezing.
I once had a Mitsubishi that the evaporator coil would ice up after more than an hour of highway driving. I would switch the compressor off for ten minutes and air flow would be restored. I could have replaced the thermistor but it is easy enough to manually control ice build-up.
Own a 2014 Hyundai Sonata GLS and having the exact same issue. Took it in turns out its an issue with these vehicles. Unfortunately my warranty was just up at 60,000 miles. 
So I will having to pay outta pocket… $7 part called a separator but looking at about $700 for labor. 