Hi I am having the oposite problem with my sprinter heater, only cold air. But what I did notice when I stripper the contols out was that the control that turns the vents was not completly opening/changing the vent from dashboard to windscreen. I had to move the vent from dashboard/feet to windscreen by hand. Yet to sort out why only cold air but if you have hot air how can it be the thermostat?
Tony
January 2010