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iMac does not power ON


I have a Power issue with my mid 2011 iMac.

I've already opened it up. I have experience with repairing electronics, etc. But now i'm stuck. On the logic board ( Main board) :

  • Diagnostic LED 1 goes ON, this seems a power-OK led to me, I measure 12v on the supply.
  • If I press the power button nothing happens.
  • I detached all peripherals, sdcardreader, camera, audio, hdd, speakers, etc. Still nothing.
  • I checked the power button, the button and cables are OK.
  • I can also measure 12v on the logic board.


  • What can keep it from powering on, what does it need?
  • Does it need memory in the banks to power on, ( LED #2) ?
  • Could it be a faulty CPU or GPU, does it need that to power on?
  • Do all the sensors have to be connected?
  • Could I detach the video card an test power without it?

The thing is I don't feel like replacing the logic board, its to expensive, I just wondered if there is any information about the "logic" which turns on the power, could I override it, so I can see where the smoke comes from ? :)..


Regards, Guido

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I know this is a little old post but Guido hit the nail on the head.

The numbered steps in the post below I appreciate, but it still tells nothing about why the board will not run the fans even.

I've replaced the power supply with brand new one as the old one would only work here and there.

Now I have replaced the logic board & power supply (new), and I only get First LED Lit, Just as always.

But now no matter what I try I can't even get power for fans or hard drive . Even swapping back out to old PS, that would power it up but black screen and hard drive spin up noise.

Tried known good working RAM. checked wires from plug input in the back of case to the board and grounded to the PS standoff to tighten it down.

So #2 is not working for me , and if you do not need it to diagnose, why is the logic board not detecting proper power from the brand new PS <or even the half working old one> on new and old Logic boards, to show me the next LED ? #3 which obviously should be on if I get #2 to light.


But the one led was the problem in the first board I took out . Sometimes on a fluke it felt like , when hitting the power button it would power on but do not dare shut it off as you can sit there for hours or minutes stressing the power button. So I bought a New Working Board from ebay hoook it all together and same exact thing except I have not got it to boot at all now. Can anyone tell me When Dry Testing a Mac board What plugs/sensors/ Devices need to be plugged in in order for video signal to be sent to screen? In other words detailed power from button to Screen as I think maybe a sensor or something is prohibiting the boot . All fans turn on and I know everything else works as I had it running a couple times before board swap . TY in advance,


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LED #1

Indicates that the trickle voltage from the power supply has been detected by the main logic board. This LED will remain ON whenever the iMac is connected to a working AC power source. 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 source 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. No RAM or faulty ram will not allow the computer to boot... but you don't need it to boot to diagnose the problems that the LED lights indicate.

LED #3

Indicates that the computer and the LCD display are communicating. This LED will be ON when the computer is turned on and video signal is being generated.

LED #4

Indicates that the computer has power but no video signal (e.g., the computer is starting up or the display has entered Sleep mode, turning off the video signal). This LED will pulse when the entire system has entered Energy Saver mode.

If this answer is acceptable please remember to return and mark it.

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Hi, so when iMac is running as normal, then all 4 LEDs should be on?


Well... No, some of the LED's don't stay on as in the case of LED#4 it will go off when in energy saver mode. Otherwise they do all stay on.


So if led #2 its blinking after pressing the power button, and the fans are spinning on and off. It's a faulty power supply?


what if you only have LED 1 lit up. what does that mean?


Your logic board has a problem ;-{


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Just by looking at the LED meanings, I think the first 3 LEDs mean your iMac is working properly. The 4th LED is only on when your device is in "sleep mode".

Hope this clears it up a little...

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I am stuck where only the first 2 LEDs are on. I just tried replacing the RAM to see if that would work. No dice. System doesn't have start up sound either. Just the fans turn on. Is that logic board? V-sync board? or LED display?


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Had the same issue. Took my imac 2011 apart and put it together and it didn't turn on only the 1st led light was showing on the logic board.

The thing that worked for me was taking the ram out and putting them back one by one. Turning the machine on each time to check that each one was working

Hope this helps someone

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I have the same symptom in this model of iMac. Just # 1 led on when computer is plugged, power button does nothing despite it shorts both power on pins properly.

Also tried removing RAM, but still fan does not spins nor beeps anyway

Reset SMC did not work either

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same here. do you know how to fix it?


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I hold my power button in for a count of ten or longer then press it again and it will reboot, not sure what error is causing this

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I've read the ENTIRE thread and can't believe someone would suggest just CHANGING the PRAM battery. Computer users are apt to consider this little piece to be eternal. That is not the case. Typically, a PRAM battery lasts up to 10 years in an iMac (depending, of course, on daily usage). But when this battery is DEAD, your Mac just refuses to start (I don't explain why here, but it should be straightforward to understand). If everything is working properly (as reported by guido), then the culprit is probably... the PRAM battery.

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guido zal eeuwig dankbaar zijn.

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