The heater will only blow cold air, so it actually isn't a heater at all. I'm wondering if the air blend damper actuator may be the problem. When the temp selector knob is turned from cold to hot, or hot to cold, I hear what I think is the sound of the blend damper moving as it should. Do I need to visually confirm its operation? There is good flow through the heater core. Could it be air locked? Is there a piece of the puzzle that I'm missing? Thanks.