Since you are experiencing no overheating problems the most likely cause is that the engine coolant temperature sending unit is faulty. This sends a signal directly to the dashboard gauge. There is another coolant temperature sensor close by which sends a signal to the ECU to help in engine management. You shouldn't get confused between the two as the one you want only has one wire and the other has two.
Here is an image showing the two sensors. The one on the left, where the single wire black connector has been disconnected is the sensor sending unit to the dashboard gauge, the one on the right with the grey connector (and being held by fingers) is the ECU sensor.
Not quite certain if my figures are correct , but to test the sending unit, disconnect the single wire black connector and using a DMM (Digital Multimeter - Ohmmeter function) measure the resistance between the sending unit terminal and earth (or ground - the engine block will be earthed). With a cold engine the resistance should be approx 140 Ohms. With a hot engine it should measure 30-50 Ohms. If you measure around this value (or definitely not near 140 Ohms) with a cold engine then the unit is faulty and needs to be replaced.
A quick test is to disconnect the lead from the sending unit, turn on the ignition and observe the temp. gauge. If it registers 'zero' then most probably the sending unit is faulty. If however it still registers a 'full scale' reading then either you have removed the wrong lead, there is a fault in the wiring or there is a fault in the instrument cluster/temp gauge
Note: Be safety aware - always switch OFF the engine when you are going to work on it