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Model A1419 / EMC 2806 / Late 2014 or Mid 2015. 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 (ID iMac15,1); EMC 2834 late 2015 / 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 (iMac17,1) All with Retina 5K displays

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On my iMac 27’’ Late 2015 #1 & #2 LED are ON (display is disconnected)

Hi.

On my iMac 27’’ Late 2015 LED#1 and LED#2 are ON (display is disconnected) and the fan starts spinning very fast after boot (the Mac makes the sound when turning on). Also you can hear a faint sizzling sound coming from the power supply before the fan starts to ramp up (because then it drowns all other noise).

The computer has been acting up the past couple of weeks, sometimes not turning on, sometimes turning on with the fans at max speed, sometimes turning off suddenly after a while.

Answer this question I have this problem too

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With the display disconnected LED #3 & 4 will be normally off.

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Here's a breakdown of the onboard diagnostic LED's:

  • LED 1 - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. The LED will remain on even when the computer has been shut down or put to sleep. The LED will turn off only if the AC power is disconnected or the power supply is faulty.
  • LED 2 - Indicates that the main logic board has detected proper power from the power supply when the computer is turned on. This LED will be ON when the computer is turned on and the power supply is working correctly.
  • 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.
  • LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.

In addition, the fan will ramp up as SMC has lost access to the displays sensors, so that’s not surprising. The faint sizzling sound needs to be investigated!

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Since there is no video card per se in the late 2015 iMac but rather the GPU is integrated on the logic board, how could the video card be installed incorrectly (per the LED 3 section here)?

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4 Comments:

@garygnu - The ‘Video Card’ is fully integrated into the logic board in this model. The LED is telling you if the data lines between the CPU and the GPU chip as well as the GPU are healthy.

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Thanks. I don't know why the LED section mentions a video card installed incorrectly or needing replacement--it's not possible, and should say something else. Will LED 3 light without the LCD connected? I don't think so, but want to make sure.

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@garygnu - It's a generic term today. It's a carry over from the original PC which offered slots to insert discrete boards (card) with the next gen systems we has MXM boards and today we have soldered on chips with there needed support chips as well as SoC's which have almost everything inside the chip! As in the case of Apples M Series systems.

LED 3 here is the GPU chip and the CPU are not communicating between. There is some support logic in between that can mess things up. One would need to get the schematics and board view drawings for the system and dive deeper into circuit to see what failed if that is where you are stuck. But you need to make sure the display is also working and the cable connections are solid. More often that's where things go bad.

But let's get a full rundown what you are encountering here as you sorta hijack someone else's question which might be different from your difficulties.

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I'm good Dan, original question was from 2022, didn't figure it was "live" anymore. we can delete all my questions, this one came up on google search. my question was answered as I reassembled my imac, 2 LEDs without LCD installed, 4 with. I just thought logically that the 3rd *should* come up even without the LCD attached, which is not correct, because apple is...different. all good, thanks for taking the time to respond

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As you have explained I think that your iMac’s behavior suggests a failing power supply unit (PSU) or logic board issue. The sizzling sound indicates possible electrical arcing or capacitor failure in the PSU. Since LED 1 and LED 2 are on, power is reaching the logic board, but the other LEDs being off means it’s not completing the boot sequence or detecting the display. Replace the power supply first, as it’s the most common failure. If problems persist, inspect the logic board for damage

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