Hello
About the 3 LED indicator lights only on a mac.
I had an issue, and I wanted to post this somewhere, since it appears that the indicator lights have multiple purposes, NOT only the commonly re-quoted meanings presented in this thread. Also, it seems like these questions come up year after year.
Hopefully, this will be of value to someone in the future.
As the original post said, the three indicator lights came on (basically) at the same time, and immediately after power being applied; then would go off and reappear.
This appears to have NOTHING to do with the staged 1-2-3-4 light sequence when the mac boots, showing communication between CPU and video.
In my case, I was playing with a 2007-2008 iMac 7,1/8,1. Everything was working fine. I was making changes, and then, I got the three LED lights on pattern described here.
I also thought it was something to do with video communication.
But, then I started going backward. Removed the video card. Applied power - 3 LED lights.
Removed the CPU. Applied power - 3 LED lights.
And that's when I realized I made a little mistake. Previously (only ONE time of many), when re-seating the CPU, I did not completely turn the CPU socket retention screw (so,. the CPU wasn't correctly attached).
A bit of an oversight. Anyway, I re-seated the CPU, and put it all back together, applied power, and it booted up.
I am leaving this here as information for others.
The mac indicator lights appear to have multiple functions. If you have a situation where 3 lights come on IMMEDIATELY on power application to the motherboard, it has nothing to do with the staged video power up sequence.
Rather, it is some much more elementary problem that is stopping the mac from completing its POST (memory, CPU, whatever).
Thanks