Be sure that the fan is not clogged with dust and that the fan duct is clean.
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You can also try to use [https://www.crystalidea.com/macs-fan-control|Macs Fan Control] to try and keep temperatures down inside your iMac.
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You can also try to use [https://www.crystalidea.com/macs-fan-control|Macs Fan Control] to try and keep temperatures down inside your iMac if you determine that the fan and fan duct are clean.