Originally Posted by
Yendor
There are two coolant temp sensors. The two wire one is for the ECU and the single wire one is for the gauge. It sounds like you are checking the wrong sensor.
Going by your symptoms I doubt it's a faulty sensor. Possibly a short to ground in the wire that runs between the gauge and the sensor or a faulty gauge.
I would start by checking for signs of damage to the engine bay wire harness. Start at the temp sensor and follow the harness. Has any work recently been done to the engine?