I was wondering if there are certain parameters that inhibit the aux heater operation (coolant temp. or ambient temp.). I was messing around with mine the other day, and it didn't appear to increase the temperature (based on a thermometer in the duct). But it was 47 degrees out and the coolant temp was past 170F. It's always worked fine when cold (both ambient and coolant temp). I was wondering if it is left on, will it shut off after the car warms up, or does it always stay on. I know some people have said it does cause a slight drop in mileage, so I am assuming it comes on whenever turned to full hot and it stays on. Just wondering . . .