In a no video situation the system may show an LED on the front of the computer there may be hard drive andor fan activity but the screen is completely black

  1. Check if the computer is sleeping. Press the space bar to wake the computer from sleep mode.
  2. Did the computer wake from sleep?

Yes: Put the computer to sleep from the Apple menu and wake the computer again to test the settings. Check Energy Saver settings to see when the computer is suppose to sleep.

No: Continue to the next step.

3. Reset your computer's PRAM (Parameter RAM).

  • If your iMac is on, turn it off by pressing the power button for five seconds. You should hear the drives and fans go quiet.
  • Wait for a few seconds.
  • While the computer is off, with your left hand, hold down the Apple key, the option key next to it, and the letter R. When you have them all pressed push the power button with your right hand and then quickly move that hand to hold down the letter P. Keep the four keys pressed while the computer starts up.
  • Make sure you hold down the four keys (P-R-Command-Option) while the computer starts up. When you keep all the keys held down, you will hear the startup chime over and over again (about every ten seconds) until you let go. After you hear the second chime, you can let go of the keys.
  • If you didn't hear at least two startup chimes, go to step one on this page and try again. It's important that you hear more than one startup chime. When you hear the second chime, you have reset your iMac's PRAM. At that point, come back to this page and we'll continue where you left off.

4. Now that you've reset PRAM, do you see video on your computer's display?

Yes: Problem resolved. Restart the computer from the Apple menu and make sure the computer is starting up properly.

No: Continue to the next step.

  1. Remove the Back Cover to access the diagnostic LEDs on the main logic board. See Removing the Back Cover for complete instructions on this procedure.
  2. With the back cover removed, locate the diagnostic LEDs 1 through 4 above the large white arrow on the main logic board.
Imac Internal Power Button
  1. Plug the power cord into the iMac G5 and into a known good AC outlet. LED #1 is should come on if you're computer has been starting up properly. If LED #1 is Off, see iMac G5: Troubleshooting No Power for further troubleshooting information.
  2. Turn the computer on by using the internal power button, the top button of the two buttons (to the left of the diagnostic LEDs).
  3. When the computer starts up, LED #2 will light after you hear the boot chime indicating the midplane is processing power correctly. If LED #2 does not light, go to step 11 for instructions on resetting the SMU. If LED #2 remains off after resetting the SMU, your midplane needs to be replaced. Order and install the midplane for your computer.
  4. LED # 3 will light last indicating that the midplane and LCD assembly have communicated and the LCD is ready to receive video. Does LED #3 come On?

Yes: This indicates that your miplane is working correctly to produce video. Continue to step 12.

No: Continue to step 11 to reset System Power Management.The replacement midplane has been installed. Do you have video now?

11. Press the SMU reset button to reset System and Power Management. This is the small metal button below the internal power button as shown.

Imac Internal Diagram

After pressing the SMU button, does LED #3 light now?

Yes: Press and hold the power button until the machine powers off. Start the machine again and confirm that LED #3 is reliably lighting. Continue to step 12.

No: If LED #3 is Off, this indicates that your midplane needs to replaced.

12. With LED #3 showing on, turn the machine off by holding in the internal power button until the machine powers off. Unplug the machine and reinstall the back cover. Return the computer to the upright position. Plug in the computer and start the computer up. Does the unit have video now?

Yes: Restart the computer and make sure that the computer is now operating properly.

No: Look closely at the display and see if it is completely black or if the video is very, very dim. Continue to next step.

13. Is the display completely black or just very, very dim?

  • Completely black. Your LCD needs to be replaced. Order and install the LCD display panel for your computer.
  • Very dim video. Your inverter board needs to be replaced. Order and install the inverter board for your computer.

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