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iMac Intel 27" EMC 2309 (Late 2009, Core 2 Duo 3.06 or 3.33 GHz) ID iMac10,1, EMC 2374 (Late 2009, Core i5 2.66 GHz or Core i7 2.8 GHz) ID iMac11,1

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iMac won't turn on. Only LED four is on in standby

iMac won't turn on. I assumed it would be the power supply but found this when I opened it up (glass and LCD apart and iMac plugged into the wall. LEDs 1, 2 and 3 are off BUT number four is ON. I've replaced power supply just to see but the same situation...any ideas? Thank you

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@macpatagon there is a possibility that you are reading the LED's from the wrong side. Besides the great answer given by my colleague, it is important to remember that those LEDs are sequential. Your iMac would have a really hard time to have LED 4 lit without the rest of them. It appears that you are looking at LED 1 which means you need to check LED Startup Sequence

LED #1 = Power available.

If LED 1 is on but LED 2 does not come on, you will need to check:

• Verify power button connection to logic board

• Verify power button functionality

  • Disconnect power button from logic board. Inspect cable and connector for damage. Verify continuity between the two pins of power button when it is pressed.

• Verify cable connection between power supply and logic board

• Verify power supply

• Verify logic board

Luckily this logic board has some test points that you can check on.

Block Image

This walks you through the steps

Block Image

Block Image

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Thanks for your answer but I am reading the leds from the correct side. I have the corresponding manual as a guide. In fact PIN number 4 on the logiboard has 12v. The only LED that lights up when the imac is connected is pin 4. The imac doesn't turn on. Very rare.


@macpatagon pin #4 is the one that corresponds to LED 1 by verifying that the psu is connected to AC power. That is good. Anyhow, appears to be a very rare error and I will have to pass on that one. I am certain there will be somebody on here that has the answer for you. Best of luck and hope you get it fixed.


Thank you, meanwhile I will continue working on this to find a solution and if so I will share it with you


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step 25,

How to find and read the Diagnostic LEDs

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Flavio zal eeuwig dankbaar zijn.

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