Fans on full when switched off

Hi,

I recently got a 21 inch iMac late 2012 second hand (from work). The iMac had Catalina installed and a 1TB fusion drive.

I was playing around with it for a week and everything was working fine, no issues at all of any kind, except sometimes it was a little slow when it hit the disk.

I decided to try and give it a boost by installing a SSD, so i removed the LCD panel, installed the SSD, put the panel back, formatted disk, put Catalina back and so far so good. Boots up, everything works and its a faster.

Except when i shut the iMac down, by selecting shutdown from the menu, the device shuts down normally but the second its off, the fans come on full blast.

Ive checked and it really is off, not in the process of shutting down with a stuck process or something, it is actually off.

If i switch it back on, the fans stop and everything is normal again. Shut it off and they come back on.

Only way to get it to stop is to physically unplug it.

Before installing the disk, everything was ok, so obviously i did something during the disk installation.

I thought the temp sensor wasnt a thing anymore, searching found me a lot of old posts from when it was new and basically said take it to Apple. Not an option anymore and it worked fine before.

Has anyone tried this recently and had a similar experience? If so, how diid you fix it?

Thanks!