High Fortigate 501E FAn RPM

I searched online but found only chassis/blade-base model 5xxx series has FAN RPM settings on the chassis.

And 1000 series or above seems to have a command to see RPMs like 1000D below:

xxx-fg1 (global) # execute sensor list

-<snip>-

19 Sys Fan 1         alarm=0  value=3200  threshold_status=0 20 Sys Fan 2         alarm=0  value=3200  threshold_status=0 21 Sys Fan 3         alarm=0  value=3400  threshold_status=0 -<snip>-

This command doesn't exist at least on 200D. I guess 50xE is the same. So you probably need to do RMA.