I made an account here to share my solution. I have a 2008 2wd 2.3L Ranger that was having the same problem. I had a code for low temp/bad thermostat. The gauge would barely go past the C. I changed the t-stat and did a coolant flush to no avail. Decided to change the Cylinder Head Temp (CHT) sensor on the top of the block between the spark plugs. Problem solved. I got one from AutoZone for 34 bucks. You pull a rubber boot off a few inches from the top of the wire to expose the plastic connector, squeeze the clip and remove the connector, and carefully put on a deep well 3/4 socket. NOTE: the socket may feel on all the way but may not be! keep rocking side to side while applying pressure until socket bottoms out and hits the metal nut of the sensor. The sensor itself has a very low torque rating so dont go crazy tightening it up, Just a little snug is fine. The old one you can break but you don't want this to happen with the new one. I hope this helps someone.

 

Welcome to FTE.

Thanks for your trouble shooting & repair info feedback & good to hear all now seems well & the engine temp gauge seems to behaving itself. Reads like you now have a spare thermostat.

Seeing as how you seem to like performing your own diagnostics & turning your own wrenches, you could have had a read here https://www.ford-trucks.com/forums/1...l#post18118173 for info on how to use your multimeter to confirm the ECT sensor resistance values were out of spec, before first tossing a thermostat at the problem.
If you don't have the inexpensive ELM scan tool, you might have a read here https://www.ford-trucks.com/forums/1...-scantool.html on it & diagnostic freeware FORScan, to run on our viewing device of choice. It's an inexpensive but powerful combo for our electronic toolbox thats tweaked for Fords & that combo can query All of our vehicle computer / controllers for trouble code clues, or have its dashboard set up to view live PID's of our choice for monitoring, so it can help focus a trouble shoot. It sure has helped me find & fix a couple of aggravating Ranger ABS & power window problems. More thoughts for consideration.