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The temp gauge was always reading cold. So I thought the T-stat went bad in the open position. I got a new OEM (Motorcraft) Thermostat, 195F and installed it. Very slight change, the temperature gauge moves a little bit but never gets in the "operating range".
I recall it wasn't doing that last year. But my heater works well....
So I think it's something else. Could it be some sensor, like coolant temperature sensor?
Could certainly be the temperature sender or guage. Warm it up and check the temperature with a candy thermometer, or an infrared heat heat gun. That will certainly be more accurate than the "idiot-light-on-a-stick" guage.