Solving PC not powering on at all

I recently solved a problem with my home PC failing to switch on. I thought I was facing a significant hardware fault, because I've never before faced a problem where the PC doesn't turn on at all.

The computer is named Theseus because it has had parts replaced for over a decade, and I wondered whether the fault was something serious — I have had a failed Voltage Regulator Module in the past, causing a melted ATX header on the main circuit board, and having to replace main board, PSU, and CPU/RAM, because my existing ones were too dated for any available "motherboards". Or maybe the problem was more benign — perhaps the 13-year-old case has a faulty power switch? But simply bridging the PWR header on the main board did not resolve the power-up failure, so I put off further diagnosis until this last weekend.

The final answer was at once simple, but also surprising! I learnt that a badly seated NVMe M.2 device can cause a short circuit on the 3.3V rail in such a way that the main board/PSU protections cause power delivery to scram, even before POST.

I honestly don't understand how the NVMe became unseated, since it is held in place by a screw. But I am happy to have found and fixed the fault without having to replace any components. This would have been difficult and expensive in the current computer economics situation.

A full diagnostic report follows, for future me.

PC Diagnostic Report

• System: Self-built desktop
• Date: March 2026
• Presenting symptom: System would not power on at all — no fans, no lights, no drives, no POST

System Specifications

Boot chain: UEFI → GRUB on SanDisk SSD → OpenSUSE or Windows (chain-loaded via GRUB)

Diagnostic Process

Phase 1 — PSU verification

• Observed brief VRM LED flash upon mains reconnect, confirming +5VSB standby rail was live
• Performed multimeter test on 24-pin ATX connector with PSU isolated (paperclip bridging PS_ON to COM/GND)
• All rails within spec: 12V, 5V, 3.3V confirmed healthy
• Result: PSU cleared

I had to count the pins on the ATX to identify each one, since the cables are not colour-coded on the MWE Bronze V2.

Phase 2 — CMOS battery

• Removed CMOS battery
• Measured CR2032 battery: 3.02V
• Attempt start without battery: system still does not start
• Result: Battery healthy, and not a contributing factor

Phase 3 — Minimum POST configuration

Stripped system to bare minimum:

• Disconnected GPU PCIe power, and removed GPU circuit board
• Removed NVMe M.2 device
• Removed one RAM bank (B2), leaving single bank in A2
• Disconnected all SATA data cables

Observations on power-on:

• System powered on — drives spooling up, fans and lights active!
• POST debug LEDs: DRAM lit briefly then extinguished (memory init OK), VGA LED remained lit
• Speaker beep code: 1 long, 2 short — video card memory failure (expected: no GPU present, Ryzen 5 3600 has no integrated graphics)

Phase 4 — GPU reseated

• Reseated GTX 1660 Ti, connected PCIe power, connected monitor
• Result: Clean single POST beep, all LEDs extinguished, monitor showed POST output
• UEFI reported settings reset (expected after earlier CMOS battery removal)
• Only complaint: no boot drive found (expected — NVMe and SATA still disconnected)
• Main board, CPU, GPU, and single RAM bank all cleared

Phase 5 — Second RAM bank

• Reseated B2 RAM bank
• Result: System continued to POST cleanly
• Both RAM banks cleared

Phase 6 — NVMe reseated into M2A

• Visually inspected NVMe and M2A slot — no visible damage
• Reseated NVMe into original M2A slot
• Result: System powered on and POSTed cleanly
• UEFI could see the ADATA NVMe but reported no bootloader — expected, as GRUB resides on the SATA SSD

Phase 7 — SATA drives reconnected

• Reconnected all SATA drives
• Set UEFI boot drive to SanDisk Extreme SSD/openSUSE
• Result: GRUB loaded, OpenSUSE booted, Windows chain-load confirmed working

Root Cause

Poorly seated NVMe M.2 device in slot M2A.

The device appeared correctly installed (clicked into connector, secured with a screw) but was likely microscopically misaligned, causing a high-resistance or intermittent connection sufficient to prevent the system from powering on.

Notes for Future Reference

• The Ryzen 5 3600 has no integrated graphics — a GPU is required for any display output
• UEFI settings are stored in SPI flash, not solely CMOS-backed RAM; the CMOS battery primarily maintains the real-time clock on this board
• The B550 Aorus Pro POST debug LEDs sequence: CPU → DRAM → VGA → BOOT; a LED remaining lit indicates that stage failed
• M.2 devices can appear correctly seated while being subtly misaligned — if symptoms point to storage, reseat before assuming device failure
• After a UEFI settings reset, SVM Mode (AMD-V hardware virtualisation, required for WSL2, VirtualBox, KVM etc.) will be disabled
  • re-enable SVM (Secure Virtual Machine) at Settings → Advanced CPU Settings → SVM Mode