The heat in my car stays hot while I am driving but when I stop at a light or slow down the heat goes very cool. Any explanation?
First, just to be clear - when you are idling there is still plenty of air blowing out of the vents? But the air is cold? And when you adjust the blower fan setting from low to hi, the blower fan will change speeds and more or less air gets blown out?
If that’s true - and plenty of air is getting blown out of the vents then you need to check your coolant/antifreeze level. If you find it low or even if you don’t, then you need to make sure all air is purged out of the system. Then you have to find out where your engine cooling system is leaking.
If you do not have low coolant or air in the system, then you probably need to have your heater core flushed.