Is my logic board dead?

hi everyone,

I am trying to fix my father in law iMac 27” 5k 2015. The mac doesn’t power on. The model is MK482LL/A, A 1419, EMC 2834. The Logic board is 661-03172 or 820-00134-A.

According to my father in law, the iMac started having problems powering on, necessitating multiple tries and sometimes unplug- replug. The problem got slowly worse, to the point where the computer would not start at all.

I disassembled the machine and looked at the diagnostic LEDs on the motherboard. Led #1 in on when the computer is plugged. When the power button is pressed, led #2 lights up for a second and turns off. No fan. no bip, no other sign of life.

I ordered a new power supply unit, installed it, same problem.

I followed the procedure ‘deep dive’ in a service manual I found online. I measured the logic board coin battery voltage, found that it was too low (~2.2 volts). I removed the logic board and replaced the coin battery. Issue was not resolved. No sign of logic board damage by visual inspection.

I am trying the next step which is to force-reset the logic board real time clock, but I cannot find the reset pads that have to be short circuited, the picture of the logic board in the service manual is different from my logic board. I have a couple of questions:

1. where are the reset pads on my model ?
2. when resetting the Real Time Clock, must the procedure be done with the iMac plugged or not ?
3. is there anything else I could try ? (except replacing the logic board). I am electronics litterate but not a technician.

happy to post a picture of the logic board if useful. Any advice appreciated.

Cheers