It could be one of several things. The sensor only tells you it is hot. It does not regulate the heat. The thermostat regulates the flow through the radiator. If it has corrosion problems, it could not work properly. If you have a low liquid level, then ther is not enough fluid to remove the heat. If you did not put antifreeze in the water, the boiling is lower and you loose water. If the radiator cap is worn, you can loose water. If your hoses are old, you can lose water, (this includes heater hoses). If you have filled your radiator up with very hard water, your radiator could be scaled up and not removing the heat well. If the water pump goes out, then it overheats quickly.Thermostat, antifreeze, coolant leaks and water pump are the most common.