Because they are broken is one reason
i change the thermostat and the temp switch but still not working
If your temperature gauge isn't working the problem is a bad gauge, grounded or disconnected wiring or, most likely, the temp sender isn't working.
probably the sensor
Check your grounding on your temp sending unit.
temperature gauge not working can be caused by a bad temp sensor in engine, a bad gauge in dash, or a bad fuse
Temp. sensor my have stopped working. Quick and fairly easy fix.
Start with checking the fuses.
Probably the temp sending unit.
Thermostat stuck open or no stat at all. Faulty temp sensor or Gauge.
bad temp sender usually. Bad gauge rarely.
Could be temperature sensor Could be gauge
that is rather unusual i would ask the dealership you brought it from and ask them and then go from there. Some vehicles have two temp sending units, one for the temp gauge and one for the ecm. The one for the gauge generally has one wire going to it, the one for the ecm may have two wires going to it.