Chosen Solution
When power is applied, LED #1 (bottom) lights. No flickering. When power button is pressed, LED’s 2, 3, & 4 light up, in order, then go back off, leaving only #1 on. Sometimes all four LED’s stay on for a short while. most of the time they fall back down as soon as I release the power button. Just now, all LED’s stayed on for close to 3 minutes before #4 turned off. After several more moments, we are back to only #1 being lit. Fans turn on. No boot occurs, and nothing is ever presented on the display. Where do I begin looking for a solution?
The first step is to verify the AC power is clean and you have proper grounding. Get a simple AC outlet tester to test the outlet you are using is properly wired and grounded as well as check the breaker panel ground connection to either the water pipe leading out of your house or the grounding rod/s, If you mess any corrosion damage you’ll need to clean it off so you have good electrical connection to the anchor point. Use a good surge suppressor or better yet a UPS, Often times systems get killed by a surge (lightening) or drops. Make sure you have all of the peripheral devices all plugged into the same surge suppressor. Your system sounds like it encountered a surge! The power supply is the first thing I would replace, you may need to replace the logic board as well. With the display not mounted #4 LED shouldn’t light.
Here’s a breakdown of the onboard diagnostic LED’s: 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.
