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This condition may be due to deposits from the engine coolant becoming trapped in the heater core.
Some 2015-2019 Ford F-150 vehicles equipped with a 3.3L, 3.5L Duratec, 3.5L EcoBoost and 5.0L engine and equipped with a block heater built on or before January 1, 2019, may exhibit a lack of heat concern from the heating ventilation and air conditioning (HVAC) system when operating the vehicle consistently in temperatures that are below – 20°C (- 4°F). This condition may be due to deposits from the engine coolant becoming trapped in the heater core.
To correct the condition on some models, flushing the cooling system and replace the heater core is required. See TSB 20-2034 for the full diagnostic procedures and flushing method.