The thermostat is a valve, and if the valve gets stuck, it could result in the heater blowing cold air. This can happen if the engine's coolant reaches a certain temperature without the thermoses opening. With the thermostat stuck, the circulation of the coolant can be affected.
Once this occurs, the heater may blow cold air. An engine could also be slow to warm up with a stuck thermostat, or it may not correctly warm up at all.
The cooling system has air in it
Air in a car's cooling system can get stuck, which also affects how the climate control system functions. When this happens, even if you have the heat turned on, the air might come out cold.
Thankfully, this is an easy thing to fix. Flushing out all the air bubbles is all that it takes to get the heat back on in this situation.
The heater core is plugged
If coolant doesn't flow through the heater core, the climate control system will only blow cold air. This is another problem with an easy solution, as once the heater core is unblocked, the heater should work again.
It is also something that you can check on your own easily. A non-contact thermometer can show if a heater core is plugged by showing whether coolant is flowing.
Coolant isn't flowing through heater core
While a plugged heater core may be the result of corrosion, a blockage can occur for a number of reasons. Lint, dirt, pine needles and more can build up in the heater box, and once this happens, your car may only blow cold air.
This is a maintenance issue that you can also take care of on your own fairly easily. All that you need to do is take out the heater box and clean out whatever junk is blocking it.
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