Chosen Solution
OK, much better! So the fan ramping up and the kernal_task running hard is all do to the lack of sensors feeding SMC with data. SMC goes into CPU Safe Mode ramping the fan up and lowering the CPU’s clocking to a crawl to prevent it from overheating. Once you get the sensors working and reconnected properly your system will be much better! So we know the person who opened the system up before you messed up the display so you’ll need a new one 27” iMac Retina 5K Display Assembly (Late ‘15), Apple P/N 661-03255 It also sounds like you have one or more antennas messed up as well here’s a listing for them: 27” iMac Retina 5K Wireless Antenna - Lower, Apple P/N 923-0066827" iMac Retina 5K Wireless Antenna - Mid, Apple P/N 923-0066727" iMac Retina 5K Wireless Antenna - Upper, Apple P/N 923-00665
Here’s the guide to put the display in iMac Intel 27" Retina 5K Display Display Replacement
