Exactly the same symptoms here. CPU fan speed not showing in the control panel, just an orange marker.
The log file is reporting the CPU fan running at around 1550 (min/max are 800/3600 respectively). Temperature is reported 42c-50c and I'm hearing an annoyingly loud fan at all times other than when iMac is asleep.
ODD & HDD fans are running at minimum values.
I followed the FAQ link in this thread. The link appears broken, but reading through the FAQs for Macs Fan Control on the website I didn't find anything specifcally relating to the above issue.
The only suggestion I see which may have been the reason for the link refers to Apple Hardware Test, to determine if there's a hardware fault with the fan/sensor.
Happy to try that, once I've dug out the install disk(s) - unfortunately I'm stuck on Snow Leopard on an early 2006 model, so the disk isn't immediately to hand! I know, maybe it's time to let the iMac go peacefully - but it's serving as my daughter's study machine, and serving her well.
MFC is clearly a great tool, and giving me some small hope that this iMac can be resuscitated - even if it means a new fan is needed.
Can't say I'm excited about disassembling it - so could someone confirm this is the necessary course of action? (subject to results of AHT of course)
Or does the Orange marker in MFC indicate soimething else I missed in the FAQs?
Thanks to all for any help given.