OS:Fedora 27, before: different Ubuntu versions.
The fan of my Lenovo Thinkpad E520 does not engage while I am logged into GNOME. The CPU (maybe GPU?) gets that hot, that I cannot lay my hand on the keyboard. If I want to cool it down, I have to reboot the machine. Then the fan gets started before Linux is booting. The activation of the fan is then luckily prolonged until the machine goes into standby or reboots. In Ubuntu I also could switch via TTY to the console (ctrl+alt+f1 - where it shows just a terminal) and the fan would start to engage and would not stop until reboot/standby
I have this issue since 2012 when I bought it new. I tried different LTS versions of Ubuntu. Now I use Fedora 27, but the problem always existed.