Hello, recently I noticed the heater in my 2011 Hyundai sonata is not as hot as it was. I didn't put too much thought behind it because it would get the job done. Due to the cold weather I've been experiencing on the east coast with single digit temperature or the teens, the heat sucks now. Someone suggested to me that it could be a heater core but when I looked online to diagnose a heater core issue, all the symptoms for a faulty heater core, experiencing none of it. Any feedback will help.