For the past month I have been troubleshooting my 2011 27" iMac not booting.
One morning after my house suffered a power failure I noticed my iMac was powered off. I tried SMC reset, pulling the power chord for 15 secs and all that stuff to no avail. The iMac would not power on, no fan noise or the boot "boing", it was just dead. The 4 LED's were observed through the bottom vents to the left of the RAM modules. With the power chord removed and then re-inserted LED #1 was observed illuminated. When the iMac power button was depressed LED #1 extinguished and would not come back on until I removed the power chord and re-inserted it. From the fault diagnosis steps that Apple provide in the repair manual the issue pointed towards the power supply PCB, so I ordered a new one as they were not that expensive. This made sense at the time as the house power failure on/off might have spiked the power supply.
After the power supply PCB arrived I swapped it out however the same fault was still there! I checked the motherboard voltages and also the power button switch was working and all was good. I also removed the RAM modules and re-fitted them one at a time but made no difference. I then removed the motherboard to have a good inspection of the motherboard under the microscope. Im not an expert and did not notice anything obvious. I decided to re-install the motherboard and connect everything up and re-check the voltages. On insertion of the power chord the iMac started its boot process and the "boing" was heard. I noticed that there were now 3 LED's illuminated. The 4th was not lit as the screen was still off. So thinking I had resolved the issue I put the screen back on and left it working.
A few days later the iMac had powered itself off again and would not boot up again. This time the LED status lights were different. On insertion of the power chord LED #1 would flash on/off and again a few seconds later then remained extinguished. It would keep doing this no matter what I tried, SMC reset etc etc…
So only thing I had done was remove the motherboard and re-insert it again and this fixed it the first time around. So I removed the screen and disconnected all connectors from the motherboard again and then re-connected them. Lo and behold it started to boot again as normal! At this point I had wished I had disconnected each connector one at a time and tested, never mind.
The iMac worked perfectly for a further week or so then powered itself off again and would not boot with the same LED #1 blinking on/off twice then remain extinguished. Great so I had the fault again so removed the screen and this time I removed each connector one at a time and re-connected it. I started with the CPU fan – fault still remained. Next I removed the SD connector from the motherboard. As soon as I plugged the power chord in the iMac began its boot sequence. Viola the SD card reader must have been causing the issues all along. I have left the SD card reader connector disconnected as I don't use it at all. So far the iMac has been running fine for over a week now.
Previous to this issue I had an issue with my Wifi network card not being able to see my 5Ghz router wifi network. After disconnecting the SD Card connector I can now see my 5Ghz network!! I haven't got any diagrams however I am assuming both the SD card and 5Ghz must be sharing some form of power supply and the SD Card has a short or something.
Hopefully the above might help someone if they are experiencing the same issues. The LED's can be a bit misleading and point to the power supply. I thought I might have two problems ie the power supply and the SD Card Reader as I had two different LED status blinking situations. I have tested my original power supply PCB since and it is fine.
All the best