I don't understand the question.
Fan off = temperature 50C
Fan on = temperature 38C
This is normal!
There are 2 routers. Very similar load (20-30% CPU). One of them never activates fan, and the other never turns it off
That's the problem. I don't understand why?
I also have another problem: the third router. It's not connected to the network, it's idle on my desk now, and his fan is constantly at 10000-11000 RPMs! Why? Why 3 different routers works in different way?
[admin@MikroTik] > /system health print
fan-speed: 10526RPM <-------------
[admin@MikroTik] > /system resource print
Edit: yeah, I can hear that 10krpm fan. It's very very loud.