The 'Fan Control application' offers other settings & could override defaults
to make fan run faster, too; what have you tried from there? ~Each selection
has some potential.
If present, does the Fusion drive appear to have failed shared parts?
How to fix a split Fusion Drive - Apple Support
//support.apple.com/en-us/HT207584
This in itself, if not a cause of overheat or present failure
of Fusion; read & set aside for now. It might be in future.
Details in article, would not be useful at this time.
(When replacement of known failed Fusion drive, there is a need to include
what is not there, a separate Sensor to control a new drive ~ which has no
fan controller sensor. This is in aftermarket upgrade kit.)
..If this may be a project you might consider, in any event make good backups
and get ready since these harddrive models with SSD too, fail as they age..
OWC In-line Digital Thermal Sensor for 27-inch & 21.5-inch iMac (Late 2009 - Mid 2010)
[Might require investigative look into hardware causes.] this is offered with Kit.
To see what replacement &/or upgrades for iMac 21.5 inch model exist:
https://eshop.macsales.com/upgrades/imac-21-5-inch-mid-2010-3.06-ghz
(Should that iMac have failing Fusion drive; may cause excessive heat too.
However due to vintage, may or not be worth service costs involved.)
Troubleshooting might be on the agenda; be sure to make more than one
backup. Beyond an external HDD dedicated to Time Machine, and others
maybe for clones of the present system, on an external enclosed HDD?
How to Upgrade iMac Hard Drive (2009, 2010, 2011): EveryMac.com
https://everymac.com/systems/apple/imac/imac-aluminum-faq/imac-intel-21.5-inch-27-inch-aluminum-how-to-upgrade-hard-drive.html
(owc macsales offers kits w/ sensor to upgrade & replace imac drives)
DIY Bundle - 1TB OWC 3G SSD and HDD Kit for 2009-2010 iMacs
https://eshop.macsales.com/item/OWC/KITIM09HE1TB/
These usually have a replacement 'internal heat sensor' as part of kit.
You may need to consult an authorized Apple service (AASP) to see
if one of these independent Apple repairers may inspect and offer
cost assessment, what they may charge to repair your old Mac.
Genius Bar Reservation and Apple Support Options - Apple
//www.apple.com/retail/geniusbar/
The old iMac may be too old for a retail Genius appointment to help
diagnose & field questions about its status; but you may ask anyway.
PS: does the headphone jack provide audio output? Maybe external
speakers may be used if that would be an option. And try an original
Apple keyboard or not the magic one?
