All that #1 LED indicates is that the power supply is receiving trickle power. Replace the power supply.
iMac Intel 27" EMC 2429 Power Supply Replacement
Here's the part you need:
iMac Intel 27" EMC 2309, 2374, 2390 or 2429 Power Supply
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.
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My problem is much the same. Maybe we can fix them both.
Problem with my iMac 27" EMC 2429, Mid 2011 (3.4 GHz, Ci7). Had it since new, May, 2011. Original symptoms (about a month ago) were, very simply, refused to boot. Perused all applicable blogs, iFixit and MacGurus posts. Seems like lots of folks have a similar problem, but few (albeit a couple…!) ever post that they have had success in bringing it back to life. Been working on and fixing Macs for almost thirty years, so I know my way around the hardware; which makes this all the more frustrating. Have proper tools, and Apple Technician Guide.
On opening the machine, the motherboard status LEDs (1-4, L-R) were 1 on, 2-4 off. Pressing power button makes no change; no. 1 stays on as long as the unit has power. I could reset the SMC and get the fans blazing. No other LEDs would light. No “obvious” bad parts; i.e. burned, bulging, or otherwise distorted or discolored parts.
Purchased a “new” third-party power supply, that looked exactly like Apple’s original (probably rebuilt, though I suppose it could actually be New). Installed that, with absolutely no change. All just like it was with the original.
All the experts say, after that, the next try is Motherboard. Somehow, I found a brand-new, sealed, Apple replacement for a great price, and quickly snagged it off eBay. About a week later, with great expectations, I installed the obviously new and truly factory-sealed motherboard. Well, I’m here, so you already know it didn’t work. But there was a change: LED 3 started showing up anytime the machine was “powered up.” On the one hand, a positive sign, yes, but where to go from here? LEDs 1 & 3 light consistently when they should, which, among other things, means the motherboard is communicating with the video card now. But LED 2 never lights (meaning it’s not booted, duh); I even hooked up the display at one point, though that isn’t a prerequisite. It still didn’t boot and LEDs 2 & 4 have not come on.
And here we are. Next, after giving this post a couple days to percolate on the forums, and assuming the genii have not posted the magic answer, my plan is to take out everything I’ve put in, check it, and put it back in, being VERY carefull that all connections are properly seated, and no wires are crossed.
Surely, SOMEONE out there has seen this problem and come up with a solution. Here’s hoping!
par Riley Werts
Please keep me informed. I didn't want to do an R&R (remove n replace) until it finally worked, if it would at all. I may as well buy a new one if that's the case. Does anyone have the voltage chart for the power supply? At least I can learn if my new power supply is working without buying another one. Then I can go from there. Mahalo (thank you)
par Kika Waialae
Update. About a month after this post, still no success. I'm just having a hard time figuring out what else it could be! Cables? Few cables spontaneously quit while undisturbed.
One thought: Would a mid-2011 iMac start up without a video card? I'm about to yank it all apart again, just to see if that makes a difference. Also, toying with replacing that same video card, as it is the famously flawed version this machine has taken such a hit for. But — it seems the problem has typically been that a bad card just made the video unusable, not that it killed the whole machine. Thoughts?
par Riley Werts