Heater blowing cold air on 2007 Dodge Magnum

I had to put some stop leak in the radiator and suddenly the heater starting blowing only cold air. What might it be?

Asked by for the 2007 Dodge Magnum
You may have plugged up the heater core, which is why some people recommend against using this stuff. Try flushing the heater core and see if it is plugged.