My heater works fine when I turn on and warm up the car. As soon as I turn the heater off, or decrease the tempurature, the heater quits and all I get is cold air. Is my thermostat shot? Is the heating unit bad? Any suggestions would be appriciated

 

It's tough to even take a wild guess without knowing exactly what kind of car you're referring to...

 

I think he said it was a SE300. T

 

bad mono-valve. Time to replace it.......