My 2017 27" iMac had the logic board replaced just over 12 months ago. It was working fine after that. I moved overseas, and it went into shipping for 3 months. On getting it back about a fortnight ago, it would not start. Where I am currently stationed, there are no Apple Stores or approved Repairers.

SYMPTOMS

• Power switch works, and fans come on.
• No startup chime.
• Nothing on display (remains black).
• If I power it down by holding power button, it turns off, but then starts back up again after a few seconds. I have to remove power cable immediately it turns off to keep it off.

DIAGNOSTICS

• I have removed the RAM. If I try to start it up without RAM, it produces the 'no RAM' beep, but symptoms remain the same.
• I have removed the display screen. LED 1 lights up when power cable is plugged in. LED 2 lights up when I press the power button. Fans run high (as expected with no display). LEDs 3 & 4 do not light up.
• There does not seem to be any obvious damage or poorly connected cables on the inside, although I have not yet taken off anything other than the display screen.

WAY AHEAD

• I cannot replace the iMac, nor can I get one shipped to me, due to where I am currently deployed.
• Is there a PRAM battery in these models that could possibly be drained down after 3 months unplugged? Could that cause the symptoms?
• Is my only option to get hold of a new logic board and replace? Sadly, that is probably more expensive than the iMac is worth.

Thanks to anyone who has any helpful advice on what to do next.

J

I thought I would update this question. Unfortunately, it is not good news.

I managed to source a second hand logic board that I was told was working.

I followed the guide, removed my old logic board, and replaced it with a new one. Same SSD, same RAM, same screen, same PSU. On startup, there are exactly the same diagnostics as above. Now, it might be that the 'new' logic board has the same graphics chip problem as my previous one. However, is there anything else that could cause this symptom?

Thanks again for your time.

J

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