lowe

EDIT: I found a solution on Github.
I'll explain briefly, (Note, I just started using Manjaro, which is my first Linux installation... I'm sorry if I'm too detailed).

First open Dolphin (or your file explorer of choice) and navigate to
/home//.local/share/plasma/plasmoids/org.kde.thermalMonitor/contents/ui/
(Note! ".local" is a hidden folder, you'll need to access is through dolphins 'Adressbar' )

in the ui folder you'll find a file "main.qml", open that and search (ctrl + f) for "systemmonitorAvailableSources"; change this row from:
property var systemmonitorAvailableSources
to
property var systemmonitorAvailableSources: []

Next time you start your computer, you'll not have to manually reload the widget.
I read this at github, here is the link:
https://github.com/kotelnik/plasma-applet-thermal-monitor/issues/53 - Jul 05 2019

I just installed this a couple days ago, I love it! Only problem (as some already pointed out) is that you have to right click the widget and then select Reload, otherwise it will only show "OFF". - Jul 05 2019