Temp gauge is usually just a wire to the gauge. If you remove the wire at the sender and put it to ground (with key in ON/Run) it should peg the gauge. If it doesn't, use an ohm meter or circuit tester to check the circuit/wire. One end at the sender end and one at the gauge input(without being connected to the gauge). Sometimes you just unplug the dash, find the correct wire and test there. If that shows good, you have another issue.