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The 2013 revamp of the desktop Mac series known as the Mac Pro.

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Mac Pro wont power on, red LED?


First, sorry for the hiatus. I've been out of the business for a while. But I'm back!

Apple couldn't tell me anything. They offered me to pay a flat $500 fee, and they would replace anything that needed to be replaced untill it worked…

I ended up just selling it for parts…

So I have a Apple Mac Pro "Quad Core" 3.7 (Late 2013) that wont power on.

I do not know the history of the machine.

I plugged it in and got nothing, no beeps. no fan spinning, no response.

I do all the normal procedures, swapped ram, tried different SSD, reset SMC, tried different power cable and outlet. None of that worked.

I took the case off and found a bright red LED shinning.

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So I started searching the internet, thinking that there MUST be some information about this red LED.... NOTHING!!!

So I proceeded to take the whole thing apart, took it down to bare bones to make sure everything was connected properly and to see if I could find any corrosion from any liquid damage or such.

While I was at it I re applied thermal paste on both the CPU and GPU.

Nothing appeared to be out of place, no damage to the board, all connections were secure.

I put it back together, powered it on, and still nothing, just the red LED...


Please, if you have any idea what could be wrong, any suggestions, or any explanation about this red LED I would LOVE to hear it.

I'm totally baffled by this one...

Thanks in advanced,


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I tried different types of RAM, Low voltage RAM, and ECC RAM... No luck.

Based on someones suggestion I tried booting without one of the two graphics cards plugged in, no luck there.

I am at a loss.

Is thing just not gonna boot for me?


Same issue. In my case, the red light was coming from the bottom of the unit, the logic board. I replaced the logic board and was back in business. Turns out, it had been dropped. Dent on the bottom and the riser stand-off screws holding the logic board in place were broken.


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Hi! Did you solve it? I have the same problem. Best regards Petter

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Same problem. Plz, help in a advice.


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Any luck?

Was just handed a mac pro with exact same problem. This was after upgrading CPU.. only a red led on psu board lights when power cord plugged in and another red LED on the bottom (round) logic board. pressing power gives no life of any kind.

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So my issue isn't exactly the same but I stumbled onto this forum post googling so thought I would at least add what I discovered.

TLDR: Make sure the power connectors are tightened down completely - you will get weird behavior that doesn't necessarily trigger the diagnostic LEDs.

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Long answer and discovery:

In my situation the machine worked fine - booted up, worked under heavy load for hours (cinebench + unigine + geekbench 5) however, when the machine would sleep and wake itself up periodically to run time machine backups etc. (whatever its doing when PowerNap is enable in Energy Settings) it would every few days crash and reboot. Sometimes I would catch it and noticed that in Macs Fan Control the fan rpm would be 0 which is suspect and made me think the machine was randomly overheating due to the fan not spinning. Sometimes the fan would self-correct and start spinning though and my thought was that maybe the Fan was going bad and so I replaced it (the entire top housing with the fan + wifi/bt card etc) but this did not fix the issue.

Next, I opened the outer case of the machine, put a magnet near the back I/O panel so I could boot the machine and started following this wiki and checking the I/O board diagnostic LED which were all fine (pressing the little button I got 7 green LEDs which is good). Next I checked the CPU Riser card/board (the board that actually has the RAM and CPU connected to it) and the LEDs were all off during boot up (there is no button for this board) so this was also good.

At this point I wasn't sure what the issue was so I put the outer case/housing back on and then put it under more stress tests (cinebench etc.). This time I noticed something weird which was a smell from the machine but I figured it was the used fan I had replaced which was a bit dusty.

I took the outer housing off again and disconnected the top housing/fan again so I could look downward into the machine. Then I booted up the machine (magnet attached etc) and noticed a yellow light now on the CPU riser card at the bottom of the machine (power problem I think) and no output to the screen. Then I realized right in front of me the little grate over the PSU connectors was actually melted!

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This concerned me given I had not messed with this grate during the fan replacement however I had self-upgraded the CPU over a year ago and had to disassemble the entire machine. When I removed the grate, I noticed that 3 of the screws were not tightened down completely AND there was melted plastic between the screw head and the flat connector, resulting in a faulty electrical connection between the PSU and CPU Riser board/card.

The weird thing about all this was that the power issues only manifested themselves when the machine transitioned from sleep to wake or from sleep to darkwake but not under full system load at least until I had replaced the fan. My guess is that these screws have been lose for a year and when I replaced the fan, I jostled it just enough to make the connectors really disconnect and result in a yellow LED power issue on the CPU riser card/board.

Anyhow, this was a weird one but ultimately my user upgrade error. Hope this helps save someone else time in the future.

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