What causes the heater to blow cold air?

2001 KIA OPTIMA
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DAN2002
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Have no heat. The heater blows cold, have changed the thermostat and still blows cold. The top heater coil hose is hot, the bottom one is cold. I believe there is a blockage. Can anyone help me?
Jan 7, 2012 at 10:35 PM
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SATURNTECH9
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Sounds like the heater core is blocked i would start by seeing if its clogged. If one is hot and the other cold something is stopping it from circulating. The manual doesnt show a heater control valve, so that leaves a clogged heater core or weak water flow from the pump. If the engine isnt running hot, then the heater core must have some kind of restriction or an air pocket in it.

See the guides below, they will help figure this out

https://www.2carpros.com/articles/car-heater-not-working

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars
Jan 7, 2012 at 10:47 PM
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BUCK121678
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Heater problem
2005 Kia Optima Two Wheel Drive Automatic

air blows but no heat and when it gets up to temp it starts to over heat except when driving at higer speeds. i replaced the thermastat last summer
Mar 14, 2019 at 5:50 PM (Merged)
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DAVE H
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check the water pump is working !
Mar 14, 2019 at 5:50 PM (Merged)
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BUCK121678
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how do you know if water pump is working or not
Mar 14, 2019 at 5:50 PM (Merged)