Have you tried cleaning the reservoir tank?
"Siphon out coolant from recovery tank. Remove hose from bottom of recovery tank and tape up with duct tape so that nothing goes into cooling system. Then tape up plastic end connector at bottom of recovery tank. Then fill to 1/2 recovery tank with water. Using brush with sponge taped around it (the long type used to clean baby bottles), reach in from cap and try to clean the area by the sensor. Do it gently. The goal is to dislodge crud from the area, not destroy sensor. Then remove tape from bottom of recovery tank to let water drain out. Do this one or two more times. Then reinstall, and pour 100% coolant into recovery tank to half inch above max. Start engine and see if it works."
Edit. This thread has some more suggestions and tests.
Click this
And if you need a new sensor, from
www.teamfordparts.com
F5RZ17B649A $11.33
Cooling - Radiator and components - Level sensor
Level sensor - 2.5 liter 1995 - 2000