Q: Heat works okay on low fan but gets cold on higher fan speeds

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When I have my blower fan on low with temperature at the warmest setting, my heat blows moderately warm air (not as hot as it should be). The heat will only get hot during a long highway trip, but not during a 20- minute commute. Either way, the air becomes cold as I turn the blower speed up. I've replaced the thermostat and exchanged the antifreeze, but this still did not solve the problem. Both in and out hoses at the heater core gets hot after a drive. What could be the problem?

My car has 119500 miles.
My car has an automatic transmission.

The heater core fins are blocked with dirt, hair, debris from inside and outside air and will not let enough transfer of heat to the air. Core may need to be replaced. The temperature control actuator may not be moving blend door all the way to heat and need replaced. Have the blend door actuator removed and check temperature door and actuator tested for proper operation. If it is okay the replace heater core.

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