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Found another dump which functioned. Unfortunately, the problems persist. Worth a try, but not related to a BIOS corruption. What's a good next step?

Thanks. Yes, that's the same model number on mine. I had no trouble flashing the chip, verified correctly. Checked all the solder connections, it ohms out properly. Are there any special steps I should do to make this clean ME version of the EFI work? Should I just dump the old data back in?

Okay, I reflashed the chip with this bin file after saving a backup. Programmed and verified successfully. Doesn't boot, only get the first diagnostic LED. Is there a chance I got the board number wrong? I was inferring it from google searching for this iMac, A1418 EMC 3069.

It's U5210 I need to reflash, right? Looking for a WSON adapter or I'll just solder wire onto chip pads I suppose. Did a little more experimenting with it. Tried heat and cold around the GPU, no major differences. It's booting into one of 3 states: 1. External monitor works, chimes, locks up...

Thank you! I'll look up how to flash this BIOS and try it out.

21.5" 2017 iMac, A1418 EMC 3069 with 820-00597 logic board. Background: computer worked fine in 2020, wasn't used until a month ago. First boot showed these symptoms: Symptoms: Power is applied. Diagnostic lights 1,2,3 all light up properly every time. 1/3 of the time LED's 4 and 5 light up...