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Car Heater Not Blowing Hot Air

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Car heater not blowing hot air because of bad ambient air sensor, heater control valve isn't opening or a problem on the blend door.

Car heater not blowing hot air caused

(1) Bad ambient air sensor.
(2) The heater control valve isn't opening.
(3) Problem with the blend door.

Car troubleshooting

If the car has an automatic control system, the possible problem is a bad ambient air sensor or a problem with the blend door.

An automatic temperature control system uses an onboard computer to maintain a constant passenger compartment temperature (as set by the driver). The computer is fed data from a sensor that measures ambient air temperature. Based on this data, a calculation is made by the computer to determine the appropriate blend door angle. The angle of the blend door essentially controls the mixture of hot and cold air, thus controlling the temperature of the air entering the passenger compartment. Two of the more common causes of this heater problem are described as follows.

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"Bad Ambient Air Sensor" - If this sensor is bad, it'll feed the computer inaccurate temperature data.  This will cause a miscalculation, resulting in a particular blend door angle that causes low-temperature heater output.

"Problem With Blend Door" - Sometimes the blend door can bind up or its control mechanism can fail. In either case, this can result in a door angle that cuts off the heat source.

What to do?
You'll need to take this vehicle to a specialist for diagnosis and repair.

If your car is equipped with the standard heater, the possible cause of the problem is that the heater control valve isn't opening.

We can understand this diagnosis by understanding how a standard automotive heater works. First off, there is the "heater core". It is located behind the dashboard vents and looks like a mini-radiator. Essentially, the sole purpose of the heater core is to emit heat. Where does this heat come from?

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Well, hot coolant from the engine is circulated through it. The fan (a.k.a. blower) blows air through the heater core, out the vents, and thus into the passenger compartment. So then, how does the heater control valve fit into all of this? It controls the flow of coolant to the heater core. If the valve is open, coolant will flow to the heater core and thus the fan will blow warm air out the vents. If closed, the coolant won't flow.

Hence, the air isn't warm. It should now be clear that since this vehicle's heater blows air that isn't warm, the heater control valve must not be opening. This is either because the valve itself is faulty or the device activating it is.

Fan Running but No Air Coming Out of Vents
It can be frustrating when you turn on your vehicle's fan, expecting a rush of cool or warm air, only to find that no air is coming out of the vents. Read more: Fan Running but No Air Coming Out of Vents

What to do?
Take this vehicle to a shop specializing in heater repair and have the mechanic inspect both the heater control valve and the device that activates the valve. Upon his diagnosis, have the necessary repairs done. In this particular situation, some mechanics may suggest replacing the heater core in addition to fixing the heater control valve problem. They'll claim the heater core is clogged and therefore is contributing to a low air temperature heater output.

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Although there may be some truth to this, nine times out of ten, fixing whatever is wrong with the heater control valve will dramatically improve heater performance. In other words, in most cases, replacing the heater core won't add that much to the heater's performance.

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