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! 🙏