Heater blows cold air while at a stop. Nissan Altima.

moondancer

My heater works great while I am driving but when I come to a stop the heater blows cold air. The Nissan Dealership said it was a valve and they replaced it, and my local garage flushed out the radiator and neither one fixed the problem. Anyone have this problem. More to the point, anyone know the solution to this problem?

Mr_Shiftright

Well it's a coolant circulation problem, so what comes to mind as possible, after the repairs already done, is:

low coolant level
air blockage in system (cooling system needs to be bled of air)
sticking thermostat
clogged heater core