Reply to a comment from Lars Berntrop-Bos

If you want to restore the temerature sensor, OWC sells a sensor on a cable to replace the internal one that was in the original HDD.

https://eshop.macsales.com/item/OWC/DIDIMACHDD09/

This is a SATA cable with a sensor you can stick to the replacement drive so the iMac can sense it temeprature and regulate the fans. 

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