I had a similar issue about a month ago that was fixed under warranty. No heat... even on milder winter days (40-50 degrees). If it was super cold, forget it, I might as well have had my window down. It never got warm. The dealer never gave me a potential root cause, but said there was a blockage and the heater core wasn't getting enough coolant. They flushed and that did the trick. They did say however, that if it reappeared in the near future, it would most likely be a heater core replacement... that would be the full on dash removal, etc. Hopefully, the problem got fixed the first time.