Why is my temperature gage not working

Try connecting an OBD II reader capable of displaying live data, specifically engine coolant temperatures, observing cold engine temperature at ignition on time before starting and/or when idling. Good readers will update coolant temps with some requiring a screen refresh to reflect a different temperature. The reader displays coolant temp signals from the body conrol module. The bcm drives the instrument panel gauges. If a reader displays correct temps then the temperature gauge may be faulty or the electronics in the isntrument panel may be faulty.