The most common causes of an automobile heater blowing cold air relate to the heater core. This unit is a small radiator the manufacturer installs inside the cabin of the vehicle behind the dash. A fan blows air through it, and heat from the engine moves into the vehicle to provide warmth. Heater core problems are often difficult to fix due to the core's location; owners should rule out other problems before tearing into the car.
In some cases, heaters do not heat because the coolant level is very low or the engine thermostat is stuck. With these problems, the driver is likely to notice the engine is overheating. Always check coolant levels when the engine is cool. If the car is running hot, do not continue to operate it until the problems are fixed. Overheating can lead to serious engine damage.
On cars with an automatic climate control system, the temperature sensors or computer that operates the heater to maintain the temperature could be defective.
Problems with the heater core that prevent it from warming the car include a diverter valve that is not working. The valve must open in order for water to circulate through the core. When vehicles are several years old, the core might have a plug. Other cores develop leaks and become less effective at warming the passengers.