After installing thinkfan, sudo pacman -S thinkfan
journalctl -p 3 -xb was showing an error concerning thinkfan, and I worked out that the thinkfan_acpi module wasn't being loaded on boot.
Adding thinkpad_acpi to /etc/modules-load.d/thinkpad.conf didn't work
What did work was editing /etc/default/grub and adding thinkpad_acpi.fan_control=1 to GRUB_CMDLINE_LINUX_DEFAULT
Then
sudo grub-mkconfig -o /boot/grub/grub.cfg