Nissan Repair: heater on nissan, head gasket, rad cap

Question
my heater is only blowing out cold air.  Water levels are fine although it is now overheating.

Answer
sounds like there is probably an air lock in the cooling system.  you will have the run the engine with the heater on and the rad cap off to bleed the air out of it.  the only question is how did the air get in the cooling system????  possible head gasket.

Marc