Regarding changing the sensor, if your radiator cap will allow you open it, squeeze the upper hose as much as possible, then tighten the cap (and hold, which is key). Then you can change the sensor out without loosing much, if any coolant. Puts a slight vacuum on the system.
I have mine come on too soon, i think my fan is a 185 on or something. I have a 180* thermostat, I like mine to run around 190* or so. I should have went with the 195 fan switch with the 180 stat. In short, it usually coincides with what temp thermostat you have.