+1 ^^^
Is the heater still blowing HOT air?
If so then yes the temp SENDER is most likely failing
As said above, on the top of the thermostat housing are TWO temp readers
ECT sensor, is 5volts and used only by the computer
ECT sender, is 12volts and used only by the dash board gauge
So not the same things, and auto parts guys will ALWAYS give you an ECT sensor FIRST, lol
Stress that you want a S E N D E R not a sensor
SENDER will have a red/white and a black/white stripe wire, thats the one you want to change
These are held in place by clips, not screwed in
Good read here on changing the thermostat housing:
https://howtoreplacefordthermostathousing.com/
These housings were known to crack and leak when they got older, pictures of the ECT units on 3rd page
If you ground the Red/white wire and turn on the key, the temp gauge should go to HOT, this tests if the wiring is OK
i.e. put a jumper wire between the red and black wires on connector, if gauge goes to HOT then both the red and black(ground) are OK