I received an iMac 27" 5K Late-2015 with a dead logic board (only the first LED indicator lit when plugged in and switched on).
I changed out the board for a pre-owned working board from the exact same model.
No problems with the install, and when I powered up the Mac (without attaching the screen), the fan started and I heard the start-up chime.
However, only the first two LEDs on the logic board were fully illuminated.
The third, on looking closely at it, had a faint green light visible at its center.
So what does the third LED tell us?
LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.
OK, that's pretty clear but it indicates that the LED has only two states: it's either ON or it's OFF.
In my case, however, the third LED was certainly not OFF but it wasn't really ON to the same extent as the first two LEDs.
Indeed, while the first two LEDs were shining brightly, the third could best be described as "glimmering".
But there's more.
When I hooked up the display (and holding slightly away from the body in order to see the LEDs on the logic board) and switched on, all four LEDs (yes, including the previously glimmering third) shone brightly and the screen showed activity (no OS on the HDD so only saw the question mark folder).
But even this didn't persist. After a minute or two, the ?-folder moved slowly to the right of the screen and the computer shut itself down.
Repeated attempts to start up the Mac occasionally gave rise to the very same screen activity and four bright LEDs.
But more and more frequently, I saw no activity on the screen and just two bright LEDs with the third merely glimmering.
Eventually, the frequency of seeing screen activity on start-up dwindled to zero and now the fourth LED never lights up and the third never goes beyond a glimmer.
Were the third LED to remain OFF, I think I could conclude from the description given above that the GPU on the board had failed.
But what information can I deduce from a faintly glimmering third LED?
I changed out the board for a pre-owned working board from the exact same model.
No problems with the install, and when I powered up the Mac (without attaching the screen), the fan started and I heard the start-up chime.
However, only the first two LEDs on the logic board were fully illuminated.
The third, on looking closely at it, had a faint green light visible at its center.
So what does the third LED tell us?
LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.
OK, that's pretty clear but it indicates that the LED has only two states: it's either ON or it's OFF.
In my case, however, the third LED was certainly not OFF but it wasn't really ON to the same extent as the first two LEDs.
Indeed, while the first two LEDs were shining brightly, the third could best be described as "glimmering".
But there's more.
When I hooked up the display (and holding slightly away from the body in order to see the LEDs on the logic board) and switched on, all four LEDs (yes, including the previously glimmering third) shone brightly and the screen showed activity (no OS on the HDD so only saw the question mark folder).
But even this didn't persist. After a minute or two, the ?-folder moved slowly to the right of the screen and the computer shut itself down.
Repeated attempts to start up the Mac occasionally gave rise to the very same screen activity and four bright LEDs.
But more and more frequently, I saw no activity on the screen and just two bright LEDs with the third merely glimmering.
Eventually, the frequency of seeing screen activity on start-up dwindled to zero and now the fourth LED never lights up and the third never goes beyond a glimmer.
Were the third LED to remain OFF, I think I could conclude from the description given above that the GPU on the board had failed.
But what information can I deduce from a faintly glimmering third LED?