MP 1,1-5,1 Mac Pro 5,1 won’t boot reliably after shutdown — suspect PSU or aging capacitors?

[jant09]

Hi everyone,

I’m having a hard time diagnosing a boot issue with my old Mac Pro 5,1. I'm hoping someone here might help shed light on what’s going on.

My setup:​

• Mac Pro 5,1 (2010), single CPU 6-core 3.33GHz (W3680)
• 24GB RAM (3 x 8GB)
• AMD Vega 56 GPU (installed via Pixlas mod in 2019)
• 4 SSDs including one NVMe on a PCIe card
• USB 3.0 PCIe card
• Dual-boot setup: macOS and Windows 10 (mostly used for gaming under Windows)

The machine has been running rock-solid for years, especially since the Pixlas mod. But recently, it developed a strange and persistent issue when booting.

---

The issue:​

• At first, I noticed random failed boots — I'd press the power button, fans spin for 2–5 seconds (normal GPU ramp-up), then the machine would shut off abruptly.
• On second try, it would usually boot normally.
• After coming back from a month-long vacation, the Mac won't boot at all — same behavior, it powers on for 2–3 seconds then shuts off completely.
• No chime, no video output.

---

What I’ve tried so far:​

• Reset PRAM/NVRAM and SMC (via unplugging power, 15s wait, replugging, etc.)
• Swapped RAM slots and modules, tried old RAM — sometimes I get a red diagnostic LED near the second RAM slot.
• I can only get a successful boot with ONE RAM stick installed (any one of them, doesn't matter which slot).
• If I boot with 1 RAM stick, leave the machine running for a while, then shut down and reinstall the full RAM, the machine boots normally again — but only if I do it immediately.
• Tested with original GPU (Radeon 5870) → no improvement, still shuts off after a few seconds.
• Tried booting with no PCIe cards, no drives, etc. — same behavior.
• Replaced the PRAM battery (CR2032).

---

My current theory:​

It feels like a PSU issue, possibly aging capacitors that can't deliver stable power at cold boot, especially with multiple RAM sticks installed. The fact that the machine boots reliably after it's been running for a few minutes (with just one RAM stick) adds weight to that theory.

Also, I experienced a shutdown while launching a game (when GPU ramps up), which again points to a power issue.

---

Questions:​

• Has anyone seen similar symptoms (boots only with 1 RAM stick / cold PSU struggles)?
• Any known PSU failure patterns in the Mac Pro 5,1?
• Could this be a deeper logic board issue instead of the PSU?
• I could try using an external ATX PSU to test — has anyone done this temporarily?

Thanks in advance for your help — I’d love to keep this beast alive a bit longer.

 

[reader50]

When you tested with no drives, did that include disconnecting the optical drive(s)? Also, did you unplug all USB devices?

Is your copy of Win10 registered? There is a known problem with Windows (when installed in EFI mode) adding security certificates to 4,1 or 5,1 firmware, which can lead to firmware corruption. This doesn't seem to happen if Win10 is not registered. Check here for more info - tsialex has a tool for downloading and analyzing your firmware for issues. Saving a backup copy of your firmware is highly recommended if you suspect corruption.

I've had Win10 installed on a drive in my system (via EFI mode) and my firmware doesn't show any Windows security certs. Presumably because the install isn't registered. You can't change the wallpaper, but there doesn't seem to be any other consequence to holding off registration.

 

[jant09]

Hey, actually I never disconnected the keyboard and mouse. And yes optical drive is unplugged.

For Win10 yes I have a licence. EOM one, but I never had any issue with it or reminder for activation. Win10 is installed since many years now

 

[krakman]

All my failed Mac Pros have died from corrosion of the metal circuitry, due to living in a high humidity environment by the sea.

I experienced similar start up issues to you.

The simplest fix is to buy another Mac Pro on Ebay and swap valuable peripherals over.