2002 Tbird Warm air of heater becomes cool

Hello, here is my story. The heater temperature can be increased by one degree at a time up to 85 degrees, and then the next jump is straight to 90 degrees. At 90 degrees, the heater always works properly, maintaining its temperature indefinitely. However, at any other temperature--85 degrees or less--warm air blows at first but after a few minutes it turns to unheated air. A couple of years ago, the Ford Dealership changed the external temperature mechanism which didn't help. Then, back on the computer, it indicated the DCCV (Dual Coolant Control Valve, Motocraft # yg-355) was bad. Recently, I had the DCCV changed but the problem remains. I have run the self test but it indicates nothing. At the end of the test it simply shows 88 degrees, etc., on both sides of the screen. I would appreciate help with this problem. I really want it fixed.

Thanks, Thomas

Welcome to the forum ThomTbird. Unfortunately I don't have any info about the latest Tbirds. If you go to the second page of the Tech help thread (at the top of the main page) and scroll down to the last post of mine, you will find a link to an auto repair section in a library. Go to the 2002 Ford Thunderbird/Service Bulletins and Recalls/Heating, A/C, Ventilation, Defogger/Lack of Cooling from Driver....(first bulletin!). It tells how to check the resistance and voltage of the DCCV. The resistance test should tell you if the DCCV is at fault.