Have a 2014 VW Jetta, which had the heater core replaced less then a year ago and it is now experiencing the same issue when it was taken to the dealer the first time. I checked all the basic potential problems. Ran the car for about 30 minutes to reach operating temperature. Had scan tool hooked up and temperature readings are within specifications. No coolant leaks found and touched all hoses which are hot. The blend knob is working properly and when at idle, do feel a slight warm breeze coming from the vents. Has anyone experienced a similar problem? Does anyone have some suggestions for me to look at? Any and all recommendations are appreciated.