I tried to upgrade one of the 2 HDDs to an SSD, and when I reassembled the Mini, it started overheating because the fan didn't come on.
Opened it up again, checked all connectors, same issue.
Put in a new fan, same issue.
Put in a new HDD sensor, same issue
Disconnected HDD sensors, same issue
Re-installed original HDD, same issue.
I've also zapped the NVRAM, and of course SMC resets after you've had it unplugged anyway.
Any ideas?