The 2014 Mac mini does not have thermal sensors which are removable from the logic board - but some can be damaged during the upgrade process (they are on the logic board which you need to remove to install a 2.5” SATA SSD)
I would suggest something like [link|https://crystalidea.com/macs-fan-control|MacsFanControl] to set the fan speed to the SSD thermal sensor (or another logic sensor which is working correctly)