most likely the heater valve is faulty. it is located in the engine compartment on the drivers side just behind the headlight. check for power to the valve with a test light. 9 times out of 10 this is your problem. they are not cheap and be a little bit of a pain to replace because the hose clamps are a little difficult to get to. if you have some difficulty you might be better off taking it to a shop. hope this helps