Could be a bad auxiliary pump. Bottom left side (driver side) of the radiator.
Usually when this one goes out, there's no heat in the cabin.
To check: Open the expansion tank cap, car in "P", key in pos2, HVAC set to your normal heat comfort.
You should hear the coolant gurgling, and also see a stream of coolant (if it's topped off) coming from the upper snaked hose that sits on top of the radiator. If nothing happens, the pump is dead and needs replacing.