Overheating Apple Support No Solution
My iMac is really overheating. often 90-102ª
Apple says nothing wrong with sensors.
Activity Monitor not too severe, so...
Have you guidance on renewing the CPU Thermal Paste.
I once had this exact issue on a G5 imac and I beleive this INTEL iMac has the some solution required.
Here's what issue I have, shown with iStat:
Make sure where you have the system the air & exhaust inlets aren't blocked if they are that could explain the higher temps. If not, follow the next steps.
Systems over time do need to be opened and cleaned of dust build-up. Using a can of air and a soft painters brush 1/2" scrub in a light motion the dust build-up on the outer & inner parts of the system and using dust wipes wipe any smooth surface. Be careful here as you don't want the brush or wipe to snag anything damaging your system.
Follow this guide iFixIt guide to remove the logic board from the system. Make sure you have ESD protection setup (mat & wrist strap which is tied to ground) so you don't damage the board when you work on it.
Once you have the logic board off carefully disconnect the heat sink by sliding it off in a twisting motion as the old paste will be holding the head sink tight to the CPU. Brush & blowout the exchanger fins and fan units. So all of the dust is removed don't apply any oil on the fan or any internal areas as over time it will attract dust making the next time much harder to clean.
Put your system together it should be cooler now.