There are several reasons why your car's heater may be blowing cold air, instead of heat. It is important to understand how the heater works in your car before you can begin to try to diagnose why it is not working. First, there is NOT an electric heater element in the heater system in automobiles, like in portable heaters used in the home. The part that transfers heat into the passenger compartment is called a heater core. A heater core can be thought of as a miniature radiator. The car engine's coolant flows through the heater core while the fan (also called a blower), blows air through the heater core fins. When air blows through the heater core fins, it is warmed, therefore warming the passenger compartment.
Start by checking the coolant. As we already said coolant is what warms the heater core. So if the coolant is low, there may not be a sufficient amount of warmth to transfer heat to the air in the passenger compartment. Since it 's cold outside, low coolant may not effect the operating temperature of the engine significantly, so you won't be alerted to a coolant problem by the temperature gage on the instrument panel. Once the coolant is full, feel the heater hoses that go through the firewall. With the engine at normal operating temperature, BOTH of the heater hoses should be hot to the touch. If only one is hot, this indicates there is is a blockage in the heater core or there is air trapped in the heater core. Using a Lisle coolant funnel is helpful in purging the air from the cooling system. If the coolant is really brown, has been neglected, or if "stop leak" has been used at some point, the heater core could be stopped up. The blockage can sometimes be unstopped by removing the heater hoses and using a garden hose with a sprayer to flush the heater core out. If neither of the hoses are hot to the touch, there could also be a malfunctioning heater control valve, if used on the model you're working on. Check for presence of a heater control valve by following the heater hoses back to the engine. Sometimes, a vacuum line could have a break causing there to be no vacuum to operate the valve. Also it should be noted that if this is your first winter with this car (and you're unfamiliar with the car's repair history), the previous owner could have by-passed the heater core because of a leak. When a heater core leaks, usually they wet the passenger side floorboard. The labor cost to change heater cores is usually expensive, so sometimes people will loop the heater hoses and by-pass a leaking heater core instead of repairing it properly.
Next check for proper airflow. After you're sure the heater core is getting hot water flow, read the following information. Doors within the heater case are either controlled by electric actuators, cables or vacuum motors. Adjust the heat control to both extremes while listening for movement of the blend door. With a cable operated heater door it's easiest to hear the door thump when it closes. If the door is not operating, find the door that controls the airflow across the heater core. If there is an electric motor that controls the door, tapping on it can sometimes make it work temporarily for testing purposes. A vacuum operated motor needs vacuum to work, so using a handheld vacuum pump for testing is usually recommended. If the vacuum motor does not hold vacuum, the diaphragm is leaking requiring replacement. To go deeper into diagnoses of the control head's function and diagnosis, specific vehicle repair manual information is needed. But hopefully the basics laid out in this article has helped to get you started on the right track.
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