heater blows cold on 2000 Ford Ranger

change the heater vaule control and thermostatbut still blow cold truck does not over heat

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2 answers
Does the heater valve open fully? When the valve is open fully engine running do the two heater hoses feel the same temperature? If not the heater core is restricted, try flushing out the heater core by disconnecting the two heater hoses and flush a garden hose both directions and see if that improves circulation. PS check that heater controls work properly in directing air to the floor /screen/ vents.
Check to make sure that the coolant is flowing through the heater core.