Hi,
I agree. If the gauge is moving from cold to hot and opposite that quickly, I suspect it is the engine coolant temp sensor.
Here are are couple links you may find helpful:
https://www.2carpros.com/articles/how-a-coolant-temperature-sensor-works
https://www.2carpros.com/articles/symptoms-of-a-bad-coolant-temperature-sensor
https://www.2carpros.com/articles/coolant-temperature-sensor-cts-replacement
If you decide to replace the coolant temp sensor, here are the directions specific to your vehicle. The attached pics correlate with the directions.
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2015 Ford Fusion FWD L4-1.5L Turbo
Engine Coolant Temperature (ECT) Sensor
ENGINE COOLANT TEMPERATURE (ECT) SENSOR
303-14A Electronic Engine Controls - 1.5L EcoBoost (118kW/160PS) 2015 Fusion
Removal and Installation
Engine Coolant Temperature (ECT) Sensor
Base Part Number: 12A648
Materials
Name Specification
Motorcraft Orange Prediluted Antifreeze/Coolant (U.S.)
VC-3DIL-B (U.S.) WSS-M97B44-D2
Removal
NOTE: Removal steps in this procedure may contain installation details.
NOTE: When releasing the cooling system pressure, cover the coolant expansion tank cap with a thick cloth.
Loosen the coolant expansion tank cap.
Loosen: 2 turn(s)
Tighten the coolant expansion tank cap.
Torque : 2 turn(s)
Pic 1
Remove the air cleaner outlet pipe.
Refer to: Air Cleaner Outlet Pipe (303-12A Intake Air Distribution and Filtering - 1.5L EcoBoost (118kW/160PS), Removal and Installation) .
NOTE: Have the new component ready to install to reduce coolant loss.
NOTE: Be prepared to capture escaping fluid with a shop towel.
Disconnect the ECT electrical connector.
Release the retaining clip.
Remove the ECT sensor.
Pic 2
Installation
To install, reverse the removal procedure.
Check the coolant level in the degas bottle and add coolant as necessary.
Material : Motorcraft Orange Prediluted Antifreeze/Coolant (U.S.) / VC-3DIL-B (U.S.) (WSS-M97B44-D2)
Pic 3
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Let us know what you find.
Joe
Images (Click to make bigger)
Wednesday, January 8th, 2020 AT 6:59 PM