I drive a 1999 F250 Super Duty with a 5.4L engine. The temp gauge has never worked since I bought it used a couple years ago. Recently I decided to try and fix this issues so first I tested the gauge as my repair book suggested I do. The gauge passed the test so the book instructed me to replace the coolant temp sender in the intake manifold. I did this, and still nothing from the gauge. I gave up for the day and just disconnected it for that time being. A couple days later I was showing my brother and I decided to hook it up. Still nothing from the gauge and I left it hooked up as we went to have lunch. A few mins into the drive the gauge spiked from nothing to full hot and the engine went into limp mode. Scanned the OBDII port for codes and got the follow:

P0117 - Engine Coolant Temperature Circuit Low Input
P1299 - Cylinder Head Overtemperature Protection Active

So now I'm considering replacing the temp sender on the cylinder head to see if it makes any difference. Does anyone have any suggestions on why my gauge isn't working? Also, does anyone know exactly where on the cylinder head the temp sender is?

Thanks
-James