LED #1 blinks when connected. iMac not turning on

If there’s an LED coming on, then the power supply works - at least, well enough to power the LED.

An LED coming on (and maybe blinking) is a code for a variety of RAM/BootROM errors. Apple used to have a support note on this, but I don’t seem to be able to find it now. However, the University of Oregon has a reference:

https://it.uoregon.edu/node/3308

UofO’s list of blink error codes matches my memory, and is as follows:

• Black screen, power LED on (solid): POST or BootROM failure
• Flashing power LED once per second: bad RAM or no RAM
• Three flashes, a pause, and three more flashes (occurs continuously): marginal RAM

If the LED is blinking, I’d try replacing the RAM. If you have no compatible replacement RAM, at least remove the RAM you have, clean off the contacts on the RAM sticks with a pink rubber eraser so they’re shiny, scrub out the RAM sockets with a soft dry toothbrush to get any gunk out of them, and try booting with the cleaned/reinstalled sticks.

It’s possible that one stick may have gone bad, while the others are still OK. Although installing in pairs is preferred for this generation, you can try each stick individually to isolate any stick that may have failed, and replace only the filed ones.

In the event that any of the followup commenters have a solid LED, the only way to repair the Boot ROM is to reinstall the operating system. You may need to reformat the internal drive as well, if the drive blocks where the old Boot ROM are physically damaged.

Your best bet is to boot from an external drive (hard drive or flash drive) with Disk Utility and an appropriate OS installer. Use Disk Utility to repair the internal drive first, then see if you can restart. If not, then try to reinstall the current OS. That will replace your damaged system software without altering your user account.

If that doesn’t work, then the safest thing is to boot from an external boot drive, clone your internal drive to a second external drive (using Carbon Copy Cloner or SuperDuper!), then erase (or reformat) the internal drive using Disk Utility, reinstall the operating system, and restore your applications/user accounts from your clone or from your last trustworthy Time Machine backup.

If you have a reliable and reasonably speedy broadband connection, it’s possible to boot your computer from an online Recovery Disk using keyboard shortcuts, and do repairs/OS reinstalls even if you don’t have an external boot drive. Different OSes can be reinstalled by different methods, depending on what OSes you’ve used before.

https://support.apple.com/en-us/HT201255

https://support.apple.com/en-us/HT204904

Maybe it's the processor, but I could not test it because it was old ...

Have similar problem. I thought it was my i7 cpu, but cpu works perfectly at another machine.

I give 12v to motherboard with my bench psu (cooling was off).

I found that the led start blinking WHEN i press cpu at the top left (around pin 221). I saw at boardview that this side is for communication with ram. With another cpu i have the same problem. I put a piece of paper to cpu pins to isolate the problem.

Even with the ram dimms off, the problem exists. Ram slots seemed perfect. Cpu slot pins seemed perfect.

edit: Possible some secondary voltage was short. Found a low resitance (can’t remember how much) at PP3V3_S0 that i didn’t like, i inject 3v to check for hot areas, didn’t found.

Then i assemble machine, boot it and works.

Tried Apple Service Diagnostics, passed all tests.

Hypotheses:

It’s magically fixed (temporary or not)

It’s something missing (still i didn’t find it)

Blow the sd reader slot for dust, i might remove some piece of wire stucked within.

I did something without my knowledge that fixed the problem.

i have similar problem, but i have solved it. Power supply is good but logic board have a short. That’s why, system is in a power cycle, so led 1 is blinking. Your system try to boot up, but there is an short on your logic board.

check the voltage rails from schematics. S0, S4, S3 and s5 there must be and short.