Insufficient coolant , trapped air in the cooling system might not have enough coolant to full fill the heater core. Check the radiator coolant level and top it off as needed.
Some cooling systems have air bleed valves built into the system to aid in the removal of air pockets. Advice: For many cooling systems, the method of removing air pockets is to start the engine and allow it to run with the radiator cap off or loose until all the air escapes from the neck of the radiator. For some systems this method is not sufficient, and the manufacturer has installed air bleed valves usually near or on the thermostat housing.
If these tips had been followed and the heater is still blowing cold air, a temperature blend door actuator may involve.
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