umm you will need a volt meter and your fuse for it should be either on the inside of the driver side near the steering wheel between were the door shuts or pop the hood and there is a fuse box on the driver side near the fire wall but mostly those are your relays. Test your voltage for the fuse, then test the gauge by removing your vinal dash board on the drivers side and there is 4 bolts i believe holding the gauge cluster in place. If all your connections are good then remove your thermostat boil water and put the thermostat in the water, if it opens its not your thermostat (the mechanical piece) if all that is in working order you have a problem with the sensor.
R.I.P. 1999 Ranger XLT 3.0, 206,000 stock drivetrain
-Totaled after accident 2009.10.30 with brand new engine and tranny
1998 Ranger XLT 4X4 4.0, 200,xxx stock drivetrain
-Still running like a champ