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hello people my intel iMac (white) will not boot when i press the power button the led says on and the fans run at maximum until i force the iMac to turn off i have tried resetting the sac pram nvram etc… can anybody help? also i have changed ram and still no luck thanks.

This answer is basically for the G5 machine. Will do better when we know your exact machine. Pop the back off. Note the two CUDA buttons the logic board. On is a reset and one a power on. Hit both with the machine plugged in. Observe the diagnostic lights in the center of the board and tell us what you see. Also examine the 24 “cans” capacitors for swelling, and/or leakage. UPDATE for intel model There are possibly diagnostic LEDs to the right of the RAM slots (sometimes they can covered by a small piece of black tape that may need to be removed). Please tell us if your have them and what they are showing. LED 1 - Indicates that the trickle voltage from the power supply is detected by the main logic board. This LED will remain ON while the iMac is connected to the AC power. 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 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. LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement. LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.

@jacobpem besides all the great point that @mayer is making, try the troubleshooting first. Go through it step by step. No Video No Video, No Boot Chime, White LED ON (Symptom 1) The iMac (17-inch Late 2006) will turn on (indicated by the front LED ON), but there is NO boot chime and No Video on the display. The faint sound of the fans, hard drive, and optical drive may also be heard.

  1. Follow instructions in the “General Information” chapter to reset the SMC. Does the computer display video after successfully resetting the SMC? Yes: Problem solved. No: Continue to next step.
  2. Verify that only supported SDRAM memory has been installed and that it has been installed correctly. Unsupported and/or defective memory can prevent the iMac (17-inch Late 2006) from booting. It may be necessary to install known good memory for testing purposes and replace any unsupported or defective SO-DIMM’s identified during this process. Does the computer display video after verifying and/or replacing the memory with known good memory? Yes: Problem solved. Verify full system functionality before return the system to the customer. No: Replace the logic board. No Video, Boot Chime heard, White LED ON (Symptom 2) The iMac (17-inch Late 2006) 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.
  3. 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.
  4. Reset your computer’s PRAM (Parameter RAM): If y – our 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).
  5. 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.
  6. 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 7. No: Continue with the next step to reset system power management (SMC).
  7. Follow instructions in the “General Information” chapter to reset the SMC. 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 7. No: Go to the next step.
  8. 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 7. 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.
  9. 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.