iMac 2017 don't turn on after switching faulty HD to SSD

Some years ago I bought this iMac 2017 21.5” from second-hand. From day one it was super slow and almost unusable. After trial and error I figured it was a problem with its HD. I connected a SSD via USB to SATA to it, set it as the startup disk and it worked fine – except for the fact that it turned off frequently by itself and restarted. From time to time the startup disk preference was forgotten and it booted to its original HD, but I was able to revert startup back to SSD. At some point last year though after that happened I was never able to revert my startup disk to my SSD. It seems that it booted on its original HD and finally the faulty HD gave up and got corrupted. I wasn't able anymore to load macOS from its original HD and set my startup disk to my external SSD. I couldn't do anything besides opening it, remove the HD and switch to the SSD, which took me very long to do. I did that today and after replugging the monitor and plugging it to power, it wasn't turning on. I'm seeing only two LEDs after pressing the power button and the fan is turning on. According to this "Diagnostic LEDs" post it means that

• "If LEDs 1 and 2 are on and LED 3 is off, then the backup battery (on the back of the logic board) may need to be reseated or the logic board may need replacement".
• I also read that "Logic board diagnostic LEDs will not progress past LED 2 with a bad CPU".
• And lastly "LED 3 Indicates that computer and video card are communicating. This LED will be ON when computer is communicating properly with video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then video card might be installed incorrectly or need replacement", however I'm not hearing the startup sound at all

I would like to try to figure out what might the issue. I've tried to turn it on with the SSD, went back with its faulty HD and with no HD / SSD. I'm always getting only LEDs 1 and 2 on. I never had any visual issues with its GPU (vertical or horizontal lines etc), but I was never really able to tell what was causing it to reboot itself since I started using it.

I'd greatly appreciate any help or suggestions on this. Thanks in advance!

@danj Hi, Dan! I just changed the iMac's battery but no dice. I still get LED 1 and 2 on, the fan spins, and nothing else. I tried with the display unplugged and plugged in. No chime sound. Is there anything else I could try? Does it make sense to try to connect to another monitor via USB c to HDMI? (In case I damaged the display connectors)

@danj I also considered doing a GPU reflow process, but since the GPU is integrated to the logic board in this iMac model, that doesn't make sense, right?

Video of the diagnostic LED: https://youtu.be/e8ju3IQdSN8

After a new disassembly I reseated the RAMs

I noticed that I was using a CR2025 battery (not 2032), I switched the Panasonic BR2032 back to the logic board (I tested it and it had some charge)

I decided to remove the heat sink to reapply thermal paste to the GPU. I cleaned it up with isopropyl alcohol 99.8 and a paper towel

I did the same to the CPU

(I hope I the CPU is seated in the right position). I also bought a USB C to HDMI converter and plugged the iMac to the TV. I powered it on with SSD, HD and no drives. I got no image in the TV.

I tried the GPU reflow route today since I had nothing left to lose. I followed this video: REFLOW GPU CHIP FIX BLACK SCREEN. I covered almost the whole logic board with 5 sheets of aluminum foil, exposing only the AMD GPU:

For the reflow I followed this guide on reddit: Guide to perform a chipset reflow (0.2) [TechSupport|new_window=true].

• Preheat at 180 °C for about 3 minutes
• Crank the heatgun up to 220 °C
• Point the gun about an inch away from the chip and heat for 1-2 minutes
• Start dialing down the temperature of the heat gun
• Keep going till you reach 40-50 °C on the heatgun
• Let it cool for about 10 minutes

I used a Black & Decker heatgun to heat the GPU

And a culinary thermometer to check the temperature:

I reapplied thermal paste to the GPU and after reassembling the iMac.... No dice. I still have the same symptoms: Only LED #1 and #2 turns on, fan starts spinning, I can hear the original corrupted HD turning on and spinining if I connect it.

With that I think I tried everything in my power to get it back to life. A second hand model iMac 2017 21.5" is sold in Brazil for $800-1200 USD (I paid $1.500 USD at the time I bought it). I would love to be able to use it, with the 16 GB upgrade I did and with SSD it could be very useful for the next 5 years at least. I'm posting this in case someone with the same model faces the same in the future.

To sum up everything I tried to get this iMac back to life:

• Changed corrupted original HD to SSD (then tried several scenarios, trying to turn it on without any drive, with the old drive etc)
• Changed PRAM battery
• Cleaned old thermal paste and applied new thermal paste to GPU/CPU
• Bought a USB-C to HDMI dongle and tried to connect it to TV (no image is seen, I tried with no drives, with SSD and with old HD)
• GPU reflow