Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
Did you still need help with this issue?
If so, lets start by removing any USB sticks, SD cards, remove anything connected to the USB ports on your imac except your Keyboard , and mouse, and monitor.
Disconnect EVERYTHING besides the keyboard mouse and monitor.
T hen, shut down the machine, unplug it from power.. Then hold the power button for 20 seconds.
Then, plug it back in and try this repair first
Shut down your Mac.Locate the following keys on the keyboard: Command (⌘), Option, P, and R.Turn on your Mac.Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.Hold these keys until the computer restarts and you hear the startup sound for a second time.Release the keys.
Then, see if the mac starts.. If it does, let me know as we have further repairs to do.
If it wont start, let me know if it shows any screens or errors and get back to me so we can carry out further steps as needed.
I strive to provide great service, and greatly appreciate a 5 star rating clicked and submitted when you are satisfied.. I always answer follow up questions.Dustin S.Senior Technician and Network Switching Systems Operator / Maintainer.
Lets try this instead..
Any change afterwords?
Just follow the steps with the power cord plugged in..
Lets try this.
Lets remove, and then reinsert the ram.. This video will show you how
If it wont work with both sticks.. Try one stick at a time.
If it still wont work or even do anything different.. Then next, we need to consider replacing the Pram battery
Its not the easiest task, but whats clear is that you are having a hardware fault.. Sometimes the steps above will cure it, it just depends on which module / hardware specifically is faulting.. But the steps above are a solid place to start trying to fix the issue.
Let me know if you get stuck or run into any issues,
At the moment, cant find a pinout. However, starting on page 35 and going through the deep dive steps of this guide, may help n arrow down the issue.
Did you follow it closely, starting with page 35 / deep dive..
Did the LED lights you seen match every step of the way for the end result of replacing the logic board?
If so, whats the exact model number of your Mac?
Allright, it sounds like you were thorough, and if thats the only led you see..
A new logic board should do the trick.. When it arrives, Ill be here if you need anything,
Not a problem, and Im not trying to spam message you.. Its company policy that I always send the last reply of each conversation.