The most common reasons a car's heater may stop working include too little coolant, a failed heater core or a faulty heater control air bend door. A coolant, such as antifreeze, is necessary for air to move through the heater core and into the car.
An improperly working heater control air bend door affects newer model vehicles. The vacuum hose that connects to the heater control air bend door should be checked first to determine if there is a break in the line. If the automobile has adequate coolant, and vacuum lines are properly connected and in good working order, the heater may not be working properly because of additives that have been used in the cooling system to stop leaks. Checking the lines for proper air flow and coolant flow is the most effective way to determine whether a vacuum or water line is blocked by additives.