iMac will not power on except fans
mac sounds with a chime and fans run but screen does not come on
Kenneth Sharp, Besides what machead3 suggest, check on these things as well:"
No Video, Boot Chime heard, White LED ON (Symptom 2)
The iMac will turn on, the boot chime can be heard, a white LED appears on the front
bezel and sounds from the fan or drive activity can be heard, but the display has no picture or color.
1. Check if the computer is sleeping. Press the space bar to wake the computer from sleep mode.
Did the computer wake from sleep?
Yes: Put the computer to sleep from the Apple menu and wake the computer again to test.
Check Energy Saver setting to see when the computer has been designated to sleep.
No: Continue to next step.
2. Reset your computer’s PRAM (Parameter RAM):
If your iMac is on, turn it off by holding the power button until the unit powers off. You
should hear the fans go quiet.
While the computer is off, with your left hand, hold down the Apple key, the Option key next to it, and the letter “R” key on the keyboard. When you have these keys all pressed down, push the power button with your right hand and then quickly move that hand to hold down the letter “P” on the keyboard. Keep these four keys pressed while the computer starts up.
Make sure you hold down the four keys (P - R - Apple - Option) while the computer is starting up. When you hear the computer’s start up chime for the second time, you can let go of the keys on the keyboard.
If you didn’t hear at least two startup chimes, go to the top of this section and begin again. It’s important that you hear two startup chimes when performing this procedure.
This indicates that you have successfully reset the computer’s PRAM (Parameter RAM).
3. Does the computer display video after successfully resetting the computer’s PRAM?
Yes: Restart the computer from the Apple menu and make sure the computer is now working correctly.
No: Continue to next step.
4. Make sure the machine is powered off; power off the machine by holding in the Power Button on the rear of the machine. Turn on the machine. Observe the white LED on the front bezel during startup; it should go out after a few moments. Did the LED go off?
Yes: If the LED went off, this indicates that the main logic board has communicated with the LCD display for video signal and your main logic board should be good. You can confirm by connecting an external monitor and checking to see that you have video out. Go to Step 8.
No: Continue with the next step to reset system power management (SMC).
5. Reset the SMC (Formerly SMU).
From the Apple menu, choose Shut Down (or if the computer is not responding, hold the power button until it turns off).
Unplug all cables from the computer, including the power cord and any display cables. Wait at least 15 seconds.
Plug the power cord back in, making sure the power button is not being pressed at the same time. Then reconnect your keyboard and mouse to the computer.
Press the power button on the back of the computer to start up your computer.
Does the computer display video after successfully resetting the SMC?
Yes: Press and hold the power button until the computer powers off. Start the computer again and confirm that front LED is reliably going off. If the computer still has no video but the LED is reliably going off, continue with step 8.
No: Go to the next step.
6. Unplug the iMac and remove the SDRAM. Replace with known good SDRAM. Plug the iMac back in and power the unit on. Does the front LED go off after a few moments now?
Yes: Replace the original SDRAM and test again. If the front LED does not go off with the only the original SDRAM installed, replace the SDRAM. If the LED is now reliably going off after a few moments, but yet you still do not have any video, go to step 8.
No: If the LED remained on, the main logic board is not communicating with the LCD panel to generate video. Replace the main logic board.
7. With the LED going off, you may have a No Backlight condition or an LCD failure. In a dim room, turn the display at an angle to you and look closely at the display. Is the display completely black or can you see a dim, purplish light or glow to the display?
Completely black: Replace the inverter
Dim purplish light or glow: Replace the LCD panel.
Hope this helps, good luck.
Connect to an external monitor to determine if this is your video card or a problem in the internal LCD display chain. If an external monitor shows nothing could be your video card/logic board. If that display works you'll have to open the machine and check the diagnostic lights to see if it's cables. backlight or LCD. Follow the tags on the right for more information.
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