Coolant sensor

Jump it with a paper clip,could be a bad sensor even though it's new. Check wiring.

 

Insp. the wire inside the plug an back into wiring looking for rub and spilt places making short inside the wires?

 

Dang coolant sensors are tricky, I observed normal level as the truck was idling...mashed on the goose-it-rod and when the RPMs were at operating range, I saw the coolant get sucked into the engine only to come back to normal when idling slowed down. I made it a practice to fill to the brim, it will expel what's not needed...

 

Is it a thread in style sensor? I've seen some that if you thread them in and leave them upside-down by mistake they will read the full reservoir as empty.

 

If it's just the two wire sensor,unplug the sensor,take a paper clip jump across the two terminals so the wires are connected.
Tape it up and secure it and go for a ride,if that cures the problem then you got a bad sensor.
If not inspect the wiring,if you see any place along the wires that goes from round to flat for no reason,that would be a place to start looking.