Not sure why you thought the CPU fan was the issue, one of your fan unit has failed. Within the fan is a tachometer which SMC uses to know the fan is working, if the tach fails the system will go into CPU safe mode which ramps the fan up and lowers the CPUs clocking to prevent if from overheating.
The best thing here is to run the onboard diagnostics to identify which fan is failing. Restart the system and press the D key to enter. Let us know what it tells you as well.
Mac startup key combinations
Here’s a listing of the iMac Intel 27" EMC 2390 Fans
OK I think found a similar case to yours Late 2010 iMac getting quarter way through booting then turning off
I’m not sure if the issue is a corrupted drive or the logic board has a deeper failure here.
SMC does interface with the North Bridge Chip so it’s possible it’s failure is causing Fan to be running hard and not correctly report it’s RPM.
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