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Hyundai Elantra heat blows hot on drivers side and cool on passenger side

Hi,
My wife drives a 2010 Hyundai Elantra with a 5speed manual trans with about 40K mi on it . We purchased the service maintenance package that covers all scheduled major and regular maintenance. A year ago they replaced the head gasket because they discovered a small oil seep. This was covered by the warranty.
Well a couple of weeks ago I noticed that the heat is fine on the drivers side and blows cool on the passenger side. This is a problem particularly when defrosting the windshield. When she brought it in for the next scheduled service a couple of days ago I asked that she mention this problem to the service people. Their response was “we need to do a cooling system flush which would not be covered by the service contract” to the tune of a couple a hundred bucks. The coolant is the transparent green it should be due to it being changed one year ago so I have my doubts about it being the heater core. Would anyone have an idea what would cause this condition? Any suggestions would be appreciated.
Thanks,
Mark

I believe your car is too new to require a coolant flush so soon. Cars that actually need coolant flushes are usually fairly old with very neglected coolant.
I’ve not actually seen your car, but I’m not entirely sure of the shop’s diagnosis.
Is your car still covered under the new car warranty?

I’ll have to agree with you on the flush. I would like the service dept. to prove what the problems is rather than just shoot in the dark. I 'm not sure about warranty I’ll have to check. I was thinking it may have more to due with the mechanical switching of the vents under the dash.

That sounds more likely to me, as well.

Does anyone else have any thoughts or encountered a similar problem?
FYI the car in the pic is my 04 not the wife’s 010