Thank you this fixed the fan issue for me!!
I did the installation of a new hard drive following the ifixit guide, afterwards I had the fan issue, I did not have the fans spinning at full speed like some people have been describing but the CPU and other components would just get hot very fast, this seemed very strange, installing Mac fan control didn't help much, as the CPU and stuff would heat up, and if I did not want to damage my computer I had to crank up the fans. I was desperate for a solution, so I purchased the kit from OWC (after reading "weddings" comment) being a bit sceptical, because I wasn't sure if installing a sensor would stop the CPU from heating up. But after the Installation everything is back to normal. The CPU stays at around 40 degrees, and the fan stays quite. I don't understand how this works, but I definitely recommend installing this sensor if you have similar issues after upgrading your hard drive, even if it means opening your iMac again ;). For the record: I have an iMac 27-inch Late 2012.
My screen was wobbling, after removing the display, and tightening the screws on the display (after removing the black case at the bottom of the display, if you slide it to the right you can pull the black case of the display) the issue was fixed.
Be very careful with these, I wasn't and two of the cables detached from the connectors!!