8TB G-RAID fan is not running, drive overheats

I have an out of warranty 8TB G-RAID (eSATA, FW800 and USB2), and the fan never runs, causing the drive to overheat badly. I have replaced the fan, but the fan still never runs. Where can I buy the correct thermal sensor to replace that - or drive motherboard, if that is what’s needed? It’s such a large drive it’s worth keeping it running.