This iMac was brought in with the description of the display not coming on when turned on + no chime. After testing we found that the computer would receive power, but never past POST or get to the point where it would chime. Furthermore, after about a minute of being on, the fans would spin up to full power.
Diagnosing the Issue
After completing the standard diagnostic step of resetting the SMC, we continued with removing the display to check the LED lights on the logic board. We wanted to double check our hunch that the computer was not booting due to a faulty GPU (graphics chip).
A Word About Removing the Display
I have seen a couple cases where people who normally work on PC’s or older Macs don’t understand that the display on this model is not removed by pulling it off with sunction cups. Rather, a special tool (available at fixit) is used to very carefully cut through the double sided tape that is holding the display on. Once that tape has been cut, the display will pull away from the computer easily. Don’t try to use a suction method on the newer thin iMacs as it will almost always result in a cracked display.
LED Diagnostic Lights
On this model iMac there are four LED lights that are visible after removing the display assembly. The first LED light shows that the computer is receiving a trickle of power from the power supply as it should when plugged in. After pressing the power button, the second light should illuminate showing that the computer is receiving full power. After a few seconds of receiving full power, the third LED will illuminate confirming that the logic board is able to talk to the GPU. Once that light is lit, very shortly after that the fourth light will light up showing that the GPU is able to send information to the display.
This particular computer had the first 2 LEDs light properly. Number 3 and 4 never lit during testing. As a precaution we tested the Logic board battery and replaced the RAM with known good RAM just to be 100% certain that the GPU was the issue.
Replacing the GPU
There are a couple different models of iMacs. In this particular model, there is no separate gracphics card that can be replaced. The GPU is soldered onto the logic board, so in order to replace the graphics chip, you either have to send the board out to somebody who is very skilled in replacing GPU chips, or replace the entire board. For this model, the cost of the board was the same as the cost for repair, so a new board it is!
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