Car blowing only cool air

Thermostat could be the problem, especially back in the day when heater controls were more basic and less hi-tech. First, if your car has a temp gauge and it reads around the middle of the heat range between cold and warm, the t-stat most likely is not the problem. Next, check the coolant. It could be low enough to the point of causing low heat. In that case you wouldn`t see antifreeze in radiator or reservoir. If low, then it can be filled and a shop can find out why it`s low. If neither of these items, it`s time for a skilled tech to take over to diagnose the problem. The control unit itself could be a problem or a related component. A plugged up heater core is a slim possibility, but those don`t happen as often as they used to.