HDD to SSD: Fan Max, CPU Temp reports 130 and 35 at the same time

I'm a repair technician with A1418 HDD->SSD Upgrade. Since I replaced the HDD, the thing is slow, the fans ramp up and it shows typical temperature sensor problem. This model doesn't have any external HDD temperature sensor, and iFixit manual also doesn't mention any of that.

All flex cables (including 2 that go to the display) are 100% ok. If I use HW monitor, CPU temperature reports 130C, which usually means a problem with a sensor (missing/damaged sensor/damaged flex cable). MacFansControl uses the very same temperature sensors and reports 35C on the very same CPU at the very same time! So obviously the temperature sensor is all OK and working. This looks like a typical for iMac random problem behavior, which seems to randomly manifest in some iMacs but not others, even if they're the same model (I had dozens go through my hands, so I know some Macs are just evil for no reason like this one). I couldn't find any exhaustive information on how I can fix this Mac bug without permanently installing MacFansControl, so that fans are not crazy during boot/boot device selection as well.

Cmd+Alt+P+R doesn't work on this computer (strange, but typical for buggy iMac).

Edit: MacOS installation process also gets stuck on it. I had to install MacOS with a MacBook onto its new drive. Tried several times.

EDIT: Not the same sensor! CPU proximity sensor reads 130. CPU PECI sensor reads 30 degrees.

If anyone had this problem and/or knows a fix, I would be very grateful. Thanks!

@danj

It was a good idea on your behalf to make me take it apart entirely. There wasn't an SSD connector - it was originally with HDD, the SSD connector was unpopulated, but on the back side of the mainboard near the ports right near the edge I noticed U5600 was torn off the pads, barely hanging on the last pad. Out of 10 pads, I tore off 3 (1 not connected; and I assume I was the source of damage, although I can't place at what moment it happened).

U5600 is responsible exactly for thermal sensing. I found schematic and boardview, it was from non-4k iMac (mine is 4k), but the board view was close enough for that part of PCB (I also lost one resistor even closer to the edge of PCB - turned out to be a pull up).

I carefully straightened chip's legs with scalpel, and restored two lost pads with jumper wire. It works correctly now! I also tore power button flex cable during speaker removal (should have checked ifixit guide), but it's not my first imac power button to tear (I'm not proud of it), so that was an easy 10 min fix (open up the button, solder on two wires, glue the button back).

I still don't understand what exactly action of mine grazed off the chip off the backside of the mainboard, but I'm glad you made me look into that. Taking apart iMac down to the last screw is never fun, so whenever possible, I avoid it.

It definitely not the way I expected (when I saw empty SSD connector pads, I kinda lost hope for a moment, and then I noticed weirdly looking chip by accident).

Problem solved.