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macrumors member
Original poster
Dec 15, 2011
Hi all, I got a replacement Mac Pro 5,1 logic board which worked for couple of days. Left the Mac Pro 5,1 alone for about two weeks and now it cant boot.

PSU clicks upon pressing the power button. About 5 seconds later, the CPUA and CPUB red led lights up briefly.

The symptoms are replicable. I've removed the stock video card, swapped in a working CPU logic board but to no avail.

Any ideas?


macrumors G3
Jun 13, 2016
It’s usually the PSU. Mac Pro PSUs are extremely complex and can’t be tested with just common tools and without load.

The simplest way to diagnose is to test your PSU with a known working early-2009 to mid-2012 Mac Pro and test a known working early-2009 to mid-2012 PSU with your non-working Mac Pro.
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