My heater is blowing cold air air conditioner works fine will not defrost window

Make sure you have antifreeze and then it could be the heater core the heater core is cheap but to replace it you have to take the whole dash board out drain the antifreeze and fix it if you take it to a shop they might charge you around 5 to 8 hundreds .

coolant level must be full. all heater/radiator hoses should be hot to touch. if the temperature hand is not in the middle of H-C after engine heats up to normal operating temperature,replace the thermostat,it is stuck open!