If you ever find that the sensors on your ESXi server show an Unknown status. You may think a reboot is your only option to get it functioning again however not true.
If you SSH to the system and restart the service using:
~ # localcli hardware ipmi sel clear ~ # /etc/init.d/sfcbd-watchdog restart
Then click the reset sensors button. Everything will start working again.
VMware ESXi 5.5 – Hardware Health Status shows “Unknown”