iMac 27 2011 - wont boot up, diagnostic LED #2 blinking

[Tom.Tom]

Hello all,

Since I've benefited a lot from this forum, I thought I'd share interesting finding coming from my recent iMac fixing - maybe it will help someone.

Recently acquired a 2011 27" iMac that wont boot. Before I received it I thought it most probably could be the PSU, but after I opened it, I was surprised to see the #1 LED lit. When pressing the power button, the #2 LED blinked for a second, fans started to spin and then everything stopped. That behavior was repeating every couple of seconds. Replaced the PSU - the same.

Started looking through the Net, replaced MoBo battery, did SMC reset - the same. Took out the MoBo, looked for any suspicious traces of blown components and cold solders - nothing like that. Put the MoBo back and tried to start it without anything connected to it - just PSU and power switch. BINGO! #2 LED came on. Now the fun started - connected couple of plugs (left side) - tried to start - #2 blinking... disconnected everything like it was before - #2 blinking... hmmmm... waited 15 minutes, tried again - #2 blinking. Took out the MoBo battery, did another SMC reset - BINGO#2. This time after connecting all plugs the MoBo was booting every time just fine.
Finally I put back the GPU and WiFi - another try - #2 blinking. Took out WiFi - still problem, took out GPU - BINGO! Put other working GPU - all good. Put back the original GPU - #2 blinking!

So, that's something I have never come across before (either with iMacs with dead GPU or searching the net). I guess the GPU was fried so much it was causing some short that made PSU or MoBo disconnecting the power... The only thing that is not clear for me is why #2 was still blinking when I had GPU out and taking out battery and resetting SMC solved it - some rubbish in SMC?

Anyway - all information I have found in the Net about blinking LED #2 was: 1) check PSU, 2) check memory, 3) check battery, 4) look for new MoBo. So, before #4 I'd add 3.5) check your GPU!

Cheers
Tom

 

[USB3foriMac]

On 2011 model, the first answer you get to any issue is "most likely GPU dead", so I doubt anyone here will be surprised like you were. Sometimes the problem is asking. Your other thread was about a dead PSU. nothing about diagnostic LEDs and the boot problem.

 

[Tom.Tom]

Well, the thing with PSU was BEFORE I got this iMac on my desk. My assumption was it's PSU, because the person who was going to send it to me said there was absolutely no reaction on the power button. I guess that was just his observation without looking into the iMac. So in preparation to fix this iMac I was looking for an alternative to original PSU, because didn't want to buy the original one BEFORE checking this myself (which was a good idea )

Now, the 2nd blinking LED issue - I have not find any hint anywhere pointing to the GPU. The other thing that misled me was the LED#2 kept blinking with everything disconnected including GPU. Only after I removed the battery and did SMC reset it started to work.

A man learns for the whole live as they say...

Anyway, I'll leave this post - maybe it will help someone else searching the net, before starting to ask.

 

[Nguyen Duc Hieu]

I guess when an MXM GPU card is inserted in its slot, the iMac SMC look at the combination as an integrated logic board => LED #2 blinks or doesn't lit up if GPU is bad in someway.
This is the same as in a PC. A bad MXM card prevented my HP 800G1 USDT from booting with some beeps, removing the card and the system booted as normal.

 

[Tom.Tom]

Yeah, but so far in several iMacs I had, the dead GPU has never caused LED #2 to blink - #1 and #2 were on and #3 + #4 were on or off and system booted either with black screen or artifacts.

That #2 blinking bay be related either to the specifics of 2011 or particular type of GPU damage.