Under temp issue P0128-(already changed t-stat)

[harrisyork]

2017 EL Limited 4WD w/HD towing package.

I have a P0128 code issue
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

BUT--before you tell me to change the thermostat let me outline what all I have done so far and where this all started.(Since temp is the issue at hand, I will tell you I live in Georgia)

-Radiator Fan blows like a Jet engine(i've read all over, people say its normal, but i think it shouldn't be)
-Car "overheated" often, not regularly, but often(especially in summer, not really ever in winter)
---I verified the temp numbers on OBD2 scanner, regularly at 220F+, sometimes up to 240 in summer
-Changed thermostat, motorcraft RT1255(little if any changes)
-Changed CHT temp Sensor(9L8Z-6G004-E)
-VOILA!!! Fixed, NO overtemping, no high running fan. Temp numbers verified on OBD2 again.
-BUT----Less than 100 miles later, P0128 code. (I think whats the big deal, but obviously car doesn't like it)
-I test the temp numbers of the Thermostat housing using an Infrared thermometer and they are the same as the CHT temps sensor numbers.(so I assume I have an actual under temp condition now).
-Car now takes a good 30-45 minutes to warm up to the 190 thermostat temp. Eventually gets there, but even in this cooler weather will cool down on drives as much as 160 degrees.
-SO, I pull the thermostat and test it in boiling water with a temperature gauge in the water and it works as advertised. Opens slowly around 185 degrees. So no Thermostat problem?

WHERE DO I GO NEXT? My only play at this point is to try another thermostat, but I want to pick other people's brains before I go changing something that I just replaced.

 

[purevw]

Assuming the thermostat is working correctly, then it is possible that it is not seated correctly in the block (or housing) and coolant is getting past it. Or the thermostat is actually not sealing when it's in the closed position. I typically use gasket shellac to hold the thermostat in the block (or housing if that is the case) during assembly, so it can't dislodge. During warmup, the upper radiator hose should only get slightly warm until the thermostat opens, then get suddenly fairly hot. Another way to tell is simply look into the radiator at startup before the engine warms. If you see coolant flowing out of the channels, then coolant is getting past the thermostat for whatever reason.

 

[Motorcity muscle]

Good advice, would put a piece of string thru the open thermostat and check it after it cools