When I put mine on Auto in the cold, it blows out of the defroster. That's great to keep the windshield clear, but useless to warm the car. All that hot air turns cold when passing over the windshield, and then it blows the cold air in my face...
There's nothing to "be sure" of. When you press the Auto button, it takes over and sets everything. Your only control is the temperature. Which is the reason why I use Manual whenever I want heat.Gotta be sure all the other ventilation buttons are off...