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iMac won't boot, black screen and no external display

Hi all,

I have a 21.5" 2019 iMac that I have acquired from eBay. When I try to boot it up I get no display (or external display) or sounds from the speakers. I carefully removed the display to realise the following:

1) it starts to boot, the second green LED comes on and then restarts itself automatically, sometimes it will stay on after a reboot, but the fans just run loud and still no display.

2) no sound from the speakers at all. Even after a PRAM reset which appears to complete successfully (reboots around the same time a typical PRAM resets takes.

3) I do occasionally get a slight electrical beep coming from the iMac on boot up or when it looses power (hold down the power button). This doesn't come from the speakers and is quiet. I expect this sound when turned off but not on start-up? Though I could be overthinking.

I am using a USB type-C hub to HDMI adaptor to test the display output externally, though I don't believe it's a display issue, but I could be wrong.

I'm thinking the processor? But I'm just going by a previous similar issue I had with an old C2D iMac. Is the processor changeable?

Any help, as always, would be greatly appreciated.

Andy.

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Here's the breakdown on the onboard diagnostic LED's

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LED 1 - Power is available.

If no LED is visible:

  • Disconnect the power cord from the computer and wait 15 seconds to reset the power supply and LED status. Reconnect the power cord and check the LED status again.
  • Verify the AC source.
  • Verify that a known-good power cord is connected. Verify the cable connection between the AC inlet and the power supply.
  • Verify the cable connection between the power supply and the logic board.
  • Verify the power supply.

LED 1 + LED 2 - Power is available and the system is turned on.

If the second LED is not visible when the power button is pressed:

  • Verify that the power button is connected to the power supply.
  • Verify power button functionality.
  • Verify the cable connection between the power supply and the logic board.
  • Verify the power supply.
  • Verify the logic board.

LED 1 + LED 2 + LED 3 - Power is available, the system is turned on, and the GPU was found.

If the third LED is not visible after the system is turned on:

  • Verify whether the fan is running when turned on (reset SMC and NVRAM, verify backup battery voltage for proper startup). If the fan is not heard, go to the "No Startup" troubleshooting flow.

LED 1 + LED 2 + LED 3 + LED 4 - Power is available, the system is turned on, the logic board is communicating with the GPU, and the internal LCD was found.
If the fourth LED is not visible after the system is turned on:

  • Verify the embedded DisplayPort cable (eDP) connections between the LCD panel and the logic board.
  • Inspect the LCD display cables for cable damage.
  • Verify external video functionality and proceed according to the result:
    • If an external display works, verify/replace the LCD panel.
    • If an external display does not work, verify/replace the logic board.

LED 1 + LED 2 + LED 3 + LED 4 + LED 5 - Power is available, system is turned on, GPU is working, logic board is communicating with the LCD panel, and the LCD panel has turned on the LCD backlight.

If the fifth LED is not visible after the computer is turned on:

Verify the eDP cable and backlight cable connections between the LCD panel and the logic board.

  • Inspect the LCD display cables for cable damage.
  • Using a flashlight, check the following areas on the panel:
  • Look in the top left corner of the panel to see if there is a dim Apple logo (the Apple menu logo).
  • Look in the center of the panel to see if there is a login screen.
  • Look at the bottom of the panel to see if there is a Dock visible.
  • If the following items are visible, there maybe something wrong with the display cables or the logic board. Proceed to the next step.
  • Replace the LCD panel, check that all the LEDs are visible, and that the backlight functions. If the computer does not function after replacing the LCD panel, replace the logic board.

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7 Comments:

Thanks Dan, looks like my only option is to change the battery and cross my fingers.

@andyc7117 - Are we talking about a laptop?? I don't think the NVRAM battery is the issue here.

I was referring to the backup battery, as stated in the LED guide -

If the third LED is not visible after the system is turned on:

"Verify whether the fan is running when turned on (reset SMC and NVRAM, verify backup battery voltage for proper startup). If the fan is not heard, go to the "No Startup" troubleshooting flow."

@andyc7117 - The NVRAM RAM backup battery very rarely fails on newer systems like yours. Sorry it was a bit of humor ;-}

Ah! 🤣 In which case I guess I'll start with the PSU, then ram and processor.

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