Fan runs on full power without regulation

Hi Guys,

when i start my monitor, the fan start to running -in my opinion- with the full power.

Okay that would be normal in the beginning BUT it dont stop ...

The fan is so loud and underneath the display there is a lot of air output on the LEFT side.

Actually i tought when i set the brightness to MINIMUM and have only my desktop open on the cinema, it will regulate the Fanspeed to a minimun so you cant hear anything of them ... but it still stays on the fan RPM level...

I dont know what to do ? Maybe is there any temperatursensor broken ??

The Monitor ist connected on an iMac 5k Late 2014.

And yes i have the usb also connected with the iMac.

Please help me :(

Thanks

Manuel