How do you fixed the radiator sensor?

I expect the radiator sensor you have in mind is the water temperature sensor, which is often not in the radiator at all, but in the engine block controlling the water flow to the radiator. They usually have a wax capsule that expands when hot, and opens the water valve. If it gets too hot, it fails. But a thermostat is quite cheap. A new gasket is a good idea too.