This is most likely being caused by a defective air temperature control blend door and or actuator. This part moves back and forth deep inside the HVAC ducting to divert air across the Heater core or the AC Evaporator core, when they fail or stick they can create this issue.
SOURCE: no heat just warm air
I think you are on top of your problem,,you dont need any one elses advice.
Complete rebuild of the cooling and heating system,, simple.
SOURCE: No Heat and Overheating in 1999 Chevy Silverado
After replacing the thermostat, you must bleed the air from the cooling system.
To bleed air from the 2.2L and 2.5L engines, remove the plug or sensor
on the top of the thermostat housing. Fill the radiator with coolant
until the coolant comes out the hole. Since the plug is made out of
steel and the thermostat housing is aluminum, it is a good idea to
apply an anti-seizing compound or Teflon® tape on the plug threads
prior to installation. Install the plug and continue to fill the
radiator. This will vent all trapped air from the engine.
Any trapped air in the heating system will have to be displaced by coolant. Once the cooling system is filled, with the radiator cap off, turn of the heater at it's highest setting. Start the engine and allow it to reach operating temp. You should see a drop in the coolant level as the air in the heating system is displaced by coolant. Add coolant to the proper level and replace the radiator cap.
Keep a close eye on the coolant level for at least the next couple of weeks. The cooling system is a "closed" system. Any significant decrease in coolant level indicates a problem.
If you have any questions, let me know.
Please take the time to rate this solution.
Drive safe and be warm.
clarkco
SOURCE: heater blows cold air have replaced the thermostat
sounds like the controller went bad use temp checker see what your temp is at inlet of heater core from hose
SOURCE: I have a 2001 1500 dodge ram it has no heat
are heater inlet outlet mix valves standard for all dodge heater cores. I have 2001 dodge cummings blows warm air not hot when gauge is 195 degrees would there be a mix valve under the dash because it is not inline outside the wall
SOURCE: heater blows cold air. I have replaced thermostat,
Not an expert by any means, but i know my ranger.. I had a small leak at the top of my radiator that still cooled the engine but didnt have enough pressure to get to the heater. The heater hoses travel up over the engine and just were not gettin anything to my heater core. That or make sure the heater control valve is operating.
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