User profile for user: Ries Pennings
Ries Pennings Author
User level: Level 1
4 points

Mac Pro (Early 2009, 8 core) won't boot, DIAG LED shows blinking 'GPU OK'...

For some reason my Mac Pro 4,1 (Early 2009, 8-core) will not boot. The DIAG LEDS shows a blinking 'GPU OK' led.


In short: The Northbridge had a broken rivet. Replaced it with bolts. Installed CPU A and my machine booted fine. Installed CPU 2, no boot... Swapped CPU's and the same result, int boots on CPU A, but withe both CPU's it will do nothing... (Applied proper cleaning and Arctic Silver 5 thermal grease with all the steps mentioned.) Both CPU sockets look pristine, no bended contacts. No other signs of damage.


I think the processor board is broke somehow. But I cannot imagine why. When the Northbridge rivet broke (the one under the CPU A heatsink; not the visible one) can that have damaged the processor board? Why can it boot with only CPU A then?


I'm willing to invest in replacing the processor board, but only if I'm sure that's the reason of my Mac not booting...

Anyone here with some more insights?

Posted on Apr 30, 2017 8:24 AM

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User profile for user: Ries Pennings
Ries Pennings Author
User level: Level 1
4 points

Apr 30, 2017 10:09 AM in response to Grant Bennet-Alder

That was my first thought also... But the GPU is OK. When booted with only the CPU A I have perfect video. Tested the Radeon in another Mac Pro; it worked. (Hacked Mac Pro 1,1; so no, no chance of swapping the PCU tray)

Also tried a newer GPU but no boot with both the processors, same 'GPU OK' blinking...


I did all the steps from the Minimum Configuration Testing (macpro_early2009_repair guide.pdf), and everything is OK until I put back the second CPU.

Also ran the Apple Hardware test when booted with 1 CPU; results in error 4SMC/1/40000000: SMC. Resetting the SMC (nor Zapping PRAM) did not helped me so far.

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User profile for user: Ries Pennings
Ries Pennings Author
User level: Level 1
4 points

Apr 30, 2017 11:49 AM in response to Grant Bennet-Alder

Both CPU chips are fine, they both work in socket A. There's no visual damage to socket B or its surrounding, so your suggestion that it can also be a defective Northbridge chip sounds very plausible.

I have no clue of this chip can be replaced? Or do I have to give up on the whole processor board and find a replacement?


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User profile for user: Ries Pennings
Ries Pennings Author
User level: Level 1
4 points

May 15, 2017 12:40 AM in response to Ries Pennings

Turns out it was the processor board. Replaced it with one I found on eBay and it boots on both CPU's again!

Happy happy joy joy!


But... on this 'new' replacement processor board are still the original plastic Northbridge-heatsink rivets. Is it wise to replace them now before they brick the board again?

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Mac Pro (Early 2009, 8 core) won't boot, DIAG LED shows blinking 'GPU OK'...

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