Repair guides and support for 27" Intel iMacs by Apple, introduced October 2009.

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Mid 2011, working fine & suddenly power button does NOTHING? PRAM bat?

I’ve scoured the forum and it seems it might be a dead PRAM battery? It was working just fine and suddenly will not turn on. I hit the power button and nothing, no response. Any suggestions?

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Maor Roffe  start with the easy stuff first. Reset your SMC and PRAM. After that let us know what your diagnostic LED’s show you. I also included the quick check and deep dive for the “no Power” situation.

Resetting the System Management Controller (SMC)

The System Management Controller (SMC) is a chip on logic board that controls all power functions. If computer is experiencing any power issue, such as not starting up, not displaying video, sleep issues, or fan noise issues, resetting SMC may resolve it. To reset SMC on an iMac (27-inch, Mid 2011):

1. From the Apple () menu, choose Shut Down (or if the computer is not responding, hold power button for approximately ten seconds until it powers off).

2. Unplug the computer’s power cord.

3. Wait 15 seconds.

4. Attach the computer’s power cord, making sure power button is not being pressed. Note: If you press the power button while inserting the power cord, the iMac will enter a mode in which the fans run at full speed.

5. Wait 5 seconds, then press the power button to turn on the computer.

Resetting Parameter RAM (PRAM)

PRAM stores certain system and device settings in a location that OS X can access quickly. Exactly which settings are stored in the computer’s PRAM varies depending on the type of computer as well as the types of devices and drives connected. To reset PRAM:

1. Shut down the computer.

2. Locate the following keys on keyboard: Command, Option (Alt), P, and R. You will need to hold these keys down simultaneously in Step 4.

3. Press power button.

4. Immediately press and hold Command-Option-P-R keys.

Important: You must press this key combination before the gray screen appears.

5. Hold down keys until the computer restarts, and you hear the startup chime a second time.

6. Release keys.

Coin Battery Removal, Testing, and PRAM Reset

1. Shut down and unplug the computer. Allow several minutes for power supply to discharge.

2. The coin battery is located on the front side of the logic board (see graphic below) and provides power for the battery-backed RAM and clock. In order to reset the PRAM remove the coin battery for 1-2 minutes.

3. Measure DC voltage on battery touching battery with red probe, and grounding with black probe. If voltage is 2.7v or less, replace battery.

4. Reinstall battery and reassemble computer.

5. Power on computer.

6. If computer starts up successfully, check for and apply the latest software and firmware


LED Functions

LED #1

• Indicates that the trickle voltage from power supply has been detected by main logic board.

This LED will turn ON when you connect the iMac to a working AC power source. The LED will remain ON as long as the computer is ON or asleep.

• When computer has been correctly shutdown, the LED#1 behavior may differ:

• - If a startup event is scheduled in System Preferences/Energy saver, LED#1 will stay ON after a correct shutdown,

• - If no startup event is scheduled in System Preferences/Energy saver, LED#1 will turn OFF and will stay OFF as long as power cord is kept connected and AC power source is present.

Disconnecting the power cord and plugging it back will turn back this LED ON , even if computer is still off.

• After disconnecting and reconnecting the AC power source, this LED could remain OFF if the AC power source is missing or disconnected , if the logic board is disconnected from the power supply or from the AC receptacle, or if the power supply board is faulty.

LED #2

• Indicates that computer is turned on. This LED will be ON as long as computer is turned on (but is not asleep) and power supply and voltage regulators are working correctly.

LED #3

• Indicates that computer and video card are communicating. This LED will be ON when computer is communicating properly with video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the backup battery (on back of logic board) may need to be reseated, or the video card might be installed incorrectly or needs replacement.

LED #4

• Indicates that computer and LCD panel are communicating. This LED will be ON when computer is turned on and video signal is being generated. If LED#4 is ON and there is no image on display, then the LCD panel, the LED backlight board, or the cables between LCD and logic board or backlight board might be installed incorrectly, or need replacement.

LED Startup Sequence

LED #1 = Power available.

If no LED is visible:

• First disconnect the power cord from computer, then reconnect it to reset the LED status, in case computer was correctly shutdown and AC was kept connected since

• Verify AC source

• Verify known-good power cord is connected

• Verify cable connection between AC inlet and power supply

• Verify cable connection between power supply and logic board

• Verify power supply

LED #1 + LED #2 = Power available, and system is powered on.

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

• Verify power button connection to logic board

• Verify power button functionality

• Verify cable connection between power supply and logic board

• Verify power supply

• Verify logic board

LED #1 + LED #2 + LED #3 = Power available, system is powered on, and video card found.

If third LED is not visible after power on:

• Verify that the MXM video card is seated properly

• Verify if boot chime is present and fans are running when powered ON (reset SMC and PRAM, reseat or check backup battery if necessary for proper boot up):

If POST boot chime is not heard, go to No Startup symptom flow,

If POST boot chime is heard, go to No Video symptom flow.

LED #1 + LED #2 + LED #3 + LED #4 = Power available, system is powered on, video card

found, and internal LCD found.

If fourth LED is not visible after power on:

• Verify internal DisplayPort cable connections between LCD panel and logic board

• Inspect LCD display cables for cable damage

• Verify external video functionality, and according to result check the following items:

If external display works then verify/replace the LED backlight board

If external display works then verify/replace the LCD panel

If external display does not work verify/replace the logic board

Symptom No Power

Unlikely cause: speakers


No Power

• No power

• No fan spin

• No startup sound

• No image on external display

• No hard drive or optical drive activity

• Caps Lock LED on wired keyboard doesn’t light when pressed.

Quick Check

1. Verify power source.

2. Verify power cord.

3. Listen closely for signs of activity from system including: rotating fans, hard drive or optical drive activity, startup sound, etc.

4. Disconnect all peripherals.

5. Reset SMC.

Here is the deep dive

Deep Dive

1. Attempt normal startup. Press power button to start up computer. Does the computer exhibit any signs of power activity, such as fan or hard drive activity, startup sound or Caps Lock LED on wired keyboard?


Go to step 2.


Go to step 4.

2. Verify display functionality.Confirm whether a video signal appears on the display. Is a video image clearly visible on the display?


Run Mac Resource Inspector (MRI) to obtain latest test results. Redirect to appropriate symptom.


Go to step 3.

3. Verify video without backlight.On a display with dim or no backlight, shine an LED flashlight through the glass on the front of the display to check for video signal. Is any video visible with flashlight?


Go to Backlight Issue / No Backlight symptom flow.


Go to No Video symptom flow.

4. Disconnect power cord.Carefully inspect the connectors between AC inlet and power supply, then DC connectors and cables between power supply and logic board, for damage or poor connections.


Reseat misconnected cable(s),or replace damaged one(s).


Replace DC power cable.

Go to step 8.

5. Inspect power button and verify if it is properly connected to the logic board.


Go to step 6.


Reconnect power button.

6. Disconnect power button from logic board. Inspect cable and connector for damage. Using a multimeter (set as ohmmeter),verify continuity between the two pins of the power button when it is pressed.


Power button is functioning correctly. Reconnect power button to logic board and go to step 7.


Power button faulty. Replace rear housing (which includes power button).

7. Reconnect power cord. Press power button and verify if system turns ON. You may alternately check for diagnostic LED # 2 to illuminate when the power button is pressed. See the section on Diagnostic LEDs for more information.


Power supply functioning. If computer still won’t boot, go to No Startup.


Replace DC power cable.

Go to step 8.

8. Press power button and verify if computer turns ON.


Issue solved with replaced DC power cable.


Replace power supply.

Go to step 9.

9. Press power button and verify if computer turns ON.


Issue solved with replaced power supply.


Reinstall power supply and replace logic board.

Let us know what you found.


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Thanks for this thoughtful reply!

I cant try out your suggestion though as my iMac will simply not turn on anymore. It just doesnt respond to the power button. Im going to attempt the PRAM battery replacement and will update shortly.



I hope this helps with your iMac.

Best wishes!


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Maor Roffe crwdns2893898:0crwdne2893898:0

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