temperature gauge not working

My boyfriend has a 1997 ford mustang, v-6 engine. the fan quit working and the temp gauge started working abnormally. The first mechanic looked at it and said it was a fuse. Replaced  the fuse and said it was fixed. No. The second mechanic said it was the cooling sensor, Replaced the sensor said it was fixed and the gauge should be normal. No. Next mechanic changed the motor in the fan and the other sensor.Yeah the fan works but not the temp gauge. Still moving abnormal. The next mechanic checked it and replaced the thermostat. He said that should make the temp gauge act normal. No! Now the temp gauge doesn’t work at all. A new fuse, fan motor, two new sensors and a thermostat you would think it works quite well. Why now is the temp gauge not working? What all needs to be checked to diagnose why the temp gauge isn’t working? Please don’t say what all these other mechanics say and tell me not to worry about it. I’m sure if you travel a distance in your car you would want everything working correctly. Especially the temp gauge. Would really hate to blow out the engine.