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Model A1419 / EMC 3070 / Mid 2017 / 3.4, 3.5 or 3.8 GHz Core i5 or 4.2 GHz Core i7 Kaby Lake Processor (ID iMac18,3) / Retina 5K display. Refer to the older iMac Intel 27" Retina 5K Display (Late 2014 & 2015) guides as the system is very similar.

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3rd LED Flashes Briefly Then Shuts Off, Need Help Diagnosing.

Hi everyone,

I recently bought an iMac 27" 5K (2017) as-is with power issues, and I’ve already opened it up for inspection (It has never been opened up before). I’m hoping to get advice from anyone experienced with this model before I spend money on parts.

When I press the power button:

  • LED 1 turns on (AC power detected).
  • LED 2 turns on after that (logic board gets power).
  • LED 3 flashes very briefly (for about half a second), and I hear a faint click sound from inside.

Then it shuts down back to just LED 1 on, no screen activity.

I also recorded a short video showing the LED sequence, so you can see exactly what’s happening.

This pattern repeats every time I try to power it up. No burning smell or visible damage when I opened it.

Would love to hear from anyone who has seen this before! I’m willing to try deeper tests if you can guide me. Thanks so much in advance! 🙏

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LED 3 on these units typically represents that the logic board and GPU are communicating properly. This LED will be on when the CPU is communicating properly with the GPU. The fact that yours is flashing for a brief second is a bit interesting, as most of the times with a video card failure it doesn't come on at all and you will just watch LEDs 1 and 2 stay on while the fans slowly ramp up. My initial thoughts are it could potentially be a defect with the graphics card, which on these models is unfortunately an integrated parts that requires logic board replacement and/or advanced microsoldering techniques to rectify. However, since it flashes and you hear a click, it is also possible that the PSU is defective in some way, or there is a loose connection somewhere. It may not be able to supply enough power to the GPU during the boot up process or to the system as a whole and shuts off as soon as more power is used to bring CPU and GPU online. This generation has a very reliable GPU, so if you wanted to start out with the cheapest component to try replacing (after verifying everything else is seated correctly), I would try replacing the PSU.

Please ensure you take all stated precautions when handling high voltage power supplies.

iMac 27" 2017 Power Supply Replacement

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