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My iMac 27 is not powering, I opened it and discovered that there is one led light on when connected to power outlet but other three led are not, I checked around the board to me everything is intact but can’t figure out why is not powering when I press the power button. Please what next should I do to rectify this issue.

All the #1 LED indicates is that there is trickle power getting to the power supply. This does not means the power supply is working. I’m 98% sure you will need to replace it. But to get you to the correct part and guide I need to know your exact machine. There are two numbers on the bottom of the stand used to identify. One is an EMC number the other a model number and looks like: Axxx If you can give us those numbers we can get you tot the part and guide.