I saw this once on one my my SX20's on a specific software release. I upgraded the units to TC6.2 and fan errors went away.
What software are your SX20’s on?
What the error message?
Some of the newer endpoints and firmware versions report fan errors shortly after booting, but this goes away after a short period of time - this is normal behaviour. When are you noticing the errors being reported?
It's also mentiioned in the release notes:
Fan status in system information on web might be misinterpreted. Although it may report “fans not running” or “can’t match [RPM] rpm” it does not mean that the system is faulty or that there is a problem with the fans. If the system is operating within the temperature specification, customer or TAC should take no action. The diagnostic messages and system information statuses will be improved.
As Wayne mentioned just treat it as a cosmetic error this is a webinterface related issue and no degradation on the performance of the device.
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