Mercedes benz, e320, 2000, rdiator fan

I have a M-Benz, E320, year 2000. Why is the radiator fan turns on every time I turn on the engine and stays running full time until I turn off the engine. Temperature is around or below 80 degrees once engine is turned on for a while. It was not doing that before. Do i need to change some sensor, regulator or so? Please explain,

Thanks,

Fanuel.