Imac 2017 power down during OS(Big sur) update ,then boot fail

[woois00]

My I mac 2017 21.5 (i5 3.0G) still black screen.
This is error procedure.

1. Try to Big sur update.
2. During Big sur update, power shut down( by my son)
3. boot fail ( black screen)
4. Logic board LED #3 off
LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.


Try to repair.

1. PRAM reset - Fail
2. PRAM battery replace - Fail


Any advice would be great. I'm basically wondering how to get some money back from these useless repairs, and how to harvest functionality from the remaining parts.

 

[bsbeamer]

Reset SMC?

Reset the SMC of your Mac – Apple Support (UK)

The SMC is responsible for managing power on your Mac. Resetting it can resolve certain uncommon issues related to power or thermal management.

support.apple.com

Can also try NVRAM three times in a row if it doesn’t work first time.

 

[woois00]

Even reset SMC , still same.

 

[bsbeamer]

Wired keyboard or wireless?
NVRAM reset 3 times in a row?

 

[Airsculpture]

Does Internet Recovery not work ?

 

[bsbeamer]

Standard Recovery available? Command + R at boot, immediately after turning on. Use wired keyboard.
Safe mode? Hold shift at start. Use wired keyboard.

 

[Airsculpture]

Or Cmd+Opt+R. I forget which one is which that I used setting up my external SSD. I think the one I used installed Big Sur as default.

 

[TwoH]

If it was turned off mid-way updating the firmware, it could have messed up the firmware for your machine.

 

[Nguyen Duc Hieu]

4. Logic board LED #3 off
LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.

Your iMac is using the iGPU (intergrated on the CPU), which is not likely to be dead. As you have already opened the iMac, I would suggest:
- Remove all removable devices like SATA, RAM and power on to see if it beeps differently.
- Put back the RAM to see if the fans still running.
- Do the PRAM reset here to see if the iMac responses (chimes)
A)
If at this level, your iMac does not response, then probably the firmware chip need to be reflashed, which can only be done by a repair shop that know this kind of trade. (de-solder the firmware chip, put it on a programer and write fresh contents to it before solder it back to the logic board).

Another option is to replace the logic board with a known to be good one.

B) If your iMac responses to the PRAM reset (hardware is OK), then you can format the SSD on another Mac, put it back to the iMac and do a fresh install of OS, or try to see if it boot from the USB installer.

If the CPU is removable, then you can test it on another compatible PC.