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Q: Heater blowing lukewarm air. Had thermostat and gasket changed plus heater core back flushed. No change.

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What else would cause this problem? The work was done at a reputable mechanical shop so I am assuming they would make sure there were no air locks and coolant is full.

My car has 92700 miles.
My car has an automatic transmission.

The heater is reliant on the engine temperature getting up to and maintaining normal operating temperature. Once this is verified then you can check both heater hoses in and out of heater core with blower on low speed and heater on full hot. The two hoses should be hot and close to same temperature. Then put blower on high speed and check temperature out of vents and check both hoses again. The temperature of inlet hose will be hot and out let hose lower temperature since blower is on high. If you have good temperature of engine then heater core flow or air flow is not good. The temperature blend door actuator and core fins blockage are the most common causes of the problem.

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