Significance of dim, very dim, third LED indicator on iMac logic board

Hi BDAqua

Thanks for your reply.

Yes, I was aware of this list explaining what each LED indicates.

However, my problem is a little more complicated.

The list you posted suggests that each LED has only two states; it's either ON or it's OFF.

In my case, when I power up the Mac, the first two LEDs shine brightly but the third is neither on nor off.

It's certainly not off as there is a faint glow from it, no more than 20% of the luminosity of the first two LEDs, and it's certainly not fully on as it's nowhere near as bright as the first two.

So what does this mean?

Is the GPU still actually communicating with "the computer" but in a more timid and withdrawn manner?

If so, is the fault with the GPU or with the part of "the computer" with which it is trying to communicate?

Note that under these circumstances, when the display is attached, the fourth LED does not light up at all and nothing appears on the screen.

Nevertheless, on at least two occasions, when the screen did activate on power up, all four LEDs, including the third, did light up brightly as would be expected for a properly functioning iMac.

Have you or anybody else experienced this dimmed LED phenomenon and can explain its significance?

Thanks for that long-shot suggestion.

Sounds like a variant of the SMC reset which I had already tried.

Anyway, had a go at your suggestion, and unfortunately, no dice.

Your comment about one or more bad caps on the logic board is a real possibility.

I didn't notice any bulging caps on the "far side" of the board before I installed it.

However, I know that there are a lot of tiny capacitors on the near (visible) side of the board.

Even if any of these were defective, I have neither the competence nor the tools to identify and swap them out.

To give an idea of how faint is the luminosity of the third LED, when the MAC is powered on with the screen detached and out of the way, looking at the board from, say, two metres away, you would think only the first two LEDs are illuminated.

However, on looking closer, it's possible to see a faint green light at the very center of the LED.

My own feeling is that the GPU on the board was "dying" when I received it as initially, but only intermittently, all four LEDs lit up and there was activity on the screen.

However, even this "activity" was short lived as after a minute or so, the image on the screen moved to the right and shortly thereafter the computer shut down.

Now, I don't even get that.

Just the first two LEDs glowing brightly with the third as described above.

Thanks but what puzzles me is that I can find no reference online to the phenomenon of a dimmed or faintly glimmering third LED on an iMac logic board.

None at all.

And even on this forum and two others where I posted a similar question, nobody has reported seeing this.

Strange.

But, then again, weird things happen to me.

Thank you.

That was a very enlightening document (sorry about the rather feeble pun).

But I think it's time to admit that this logic board has had its day and I'll have to find something else to worry about.