Auxiliary fan works constatly, why?
 

99 % experienced with opposite situation, it does not work.
525 E39 September 2000
The story is next:

Once, I got "Engine Light ON" and a little bit later I realized noise which comes from full speed running fan. Temperature outside just above freezing mark I drove on HWY. Next few days it starts run just after 5 minutes drive.
I went to service. They replaced thermostat and clear fault code, told me that I had only code regarding thermostat and Bank 1.
I made some kind of research and find out that has only 3 temperature sensors:
1 - on somewhere on engine block. Really possible it created trouble, but where it located and how I can check it at home?
2 - on lover housing radiator.It is responsible for higher temperature. Also how I can check it at home?
3 - located on thermostat. It is brand new!

What case of problem and what solution?