iMac fan speed
iMac OS 13 fan runs on high continuously
iMac 21.5″, macOS 10.13
Question marked as
Have you blown the dust out lately?
Have you done a PRAM reset, CMD+Option+p+r...
http://support.apple.com/kb/HT1379
In fact, do 3 in a row, takes a bit of time.
Intel-based Macs: Resetting the System Management Controller (SMC)...