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Your EX250's temperature sensor, the little item that is screwed into the thermostat housing, is just a "thermistor" which is an electrical resistor whose resistance is greatly reduced by heating. The hotter the thermistor gets the more amperage it will allow to flow to ground (the EX250 temperature sensor is an "old school" type of sensor. It is a 1-wire sensor which means it grounds directly through the metal body of the thermostat housing into the bike's frame).
Your EX250's temperature gauge is a "Galvanometer" (which is a type of ammeter). Movement of the Temperature gauge needle is caused by amperage flowing through the instrument to the variable ground provided by the Temperature sensor thermistor in the thermostat housing. A factory-calibrated amount of electricity (DC current) flows through a small set of copper windings that when energized act as little electromagnets to exert a pull-force against the return (to zero) spring-loaded Temperature gauge needle.
So you want to make sure of the following:
1. that the Temperature gauge in the bike's instrument binnacle has good/correct voltage input to it (it's 12volts like everything else on the Pre-Gen EX250). You can test the wire with a multimeter.
2. that the Temperature sensor is correctly installed in the thermostat housing and that the threading it is screwed into isn't corroded (which would inhibit proper grounding).
3. that the wire running from the Temperature gauge in the instrument binnacle to the Temperature sensor in the thermostat housing is firmly attached at both ends. Also make sure that the attach-point on the Temperature sensor hasn't been bent so much that the metal is breaking (it's made of soft metal and is the most delicate part in the circuit).
4. that the thermostat housing is firmly bolted to the bike's frame. If it isn't, if the thermostat housing is just hanging loose on its coolant hoses, there won't be a proper pathway for the grounded power from the Temperature sensor.
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