iMac fan starts after shutdown

Hello.

I changed the hard drive (SSD) on my iMac EMC 2544, Late 2012 13,1. After that, I had exactly the same problem. Can you advise me of anything? After installing the old hard drive back, nothing has changed.(((

This is an old answer and incomplete as the OP never told me if he had changed the hard drive When changing hard drives on this model you have to address the temperature sensor as the on Apple up in was proprietary. OWC has a solution. Here it is:

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

From OWC Dyllon: Great question! The Late 2009 – current iMacs will need the OWC In-Line Digital Thermal sensor with the exception of the 21.5″ 2012-Current iMacs!

What is confusing you here is Apple altered the drive to a 2.5" in the 'Thin Series' 21.5" iMac which came out in 2012. Only the newer 'Thin Series' 27" iMac which have a 3.5" HD drive need the in-line sensor. This is if you change out the drive with either a HD or SSD in any of the 'Thin Series' 27" models

The older 21.5 & 27" from 2009 to 2011 use the 3.5" HD drives and they all need a thermal sensor either using the Apple custom cable in the 2009 & 2010 when replacing the HD with a new one. Or, the OWC in-line sensor from 2009 thru 2011 with either a HD or SSD.