Leave the drivers door open, put the key in, turn to on position (not running) and hold down the reset button (Trip) for five seconds.
If it gets to hot the thermostat wont work. you need to repace it and the gauge will work again.
only 3 reasons a temp gauge wont work the temp sender is bad the gauge is bad or the wire from temp sender to gauge is broken
check your ground wire to the gauge
fuel gauge quit working on 1997 buick park ave. what is it?
why wont 82 Chevy van fuel guage work
The sending unit is likely faulty
If your thermostat isn't working then the gauge on the car won't work. You may also notice that your car runs hot.
Check coolant sensor for corrosion at the wires or the sensor itself . If corrosion, replace sensor or repair wires.
Bad temperature sending unit? Bad gauge?
clock spring
If the temp gauge no longer works then either the gauge itself has failed, the sending unit has failed, or there is a problem in the circuitry.
Can be a blown fuse, defective sending unit, or loose of disconnected wiring. The gauge itself can be bad, but not likely.