Rubin Shrestha
Rubin Shrestha
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Why does my PC turn on but no display, no USB power, CPU LED stuck red?

by |Published

I’m facing a hardware issue that I haven’t been able to resolve despite extensive troubleshooting. I hope someone here can help me identify the root cause.

System Specifications

  • Motherboard: MSI B550 Gaming Plus
  • CPU: AMD Ryzen 5 3600 (no integrated graphics)
  • GPU: MSI RTX 5070
  • RAM: 2 × 16 GB DDR4
  • PSU: MSI MAG A750GL PCIE5 (modular)

A few weeks ago, I replaced both the GPU and the PSU. The system worked immediately after the upgrade.

I didn’t use the PC for about 3 weeks, and now it no longer boots.

Symptoms.

When pressing the power button:

  • All fans spin (CPU, case, GPU).
  • The CPU debug LED stays solid red on the motherboard.
  • The monitor wakes up, but shows no signal (DisplayPort).
  • Keyboard and mouse have no power (no LEDs).
  • No other debug LEDs (DRAM/VGA/BOOT) light up.

The system appears to power on but never reaches POST.

What I have already tested.

  1. Power supply connections.

    • The 24-pin ATX connector is fully seated.
    • The 8-pin CPU (EPS) cable is:
      • Fully inserted into the motherboard’s CPU_PWR1 port.
      • Connected to a CPU and PCIe port on the PSU.
      • Reseated the EPS cable multiple times, ensuring it clicks fully into place.

  2. GPU

    • GPU is fully seated in the PCIe slot.
    • Power connectors attached correctly.
    • Tried booting without the GPU (just to observe debug LED behaviour).
      Still CPU LED solid red.

  3. RAM

    • Tested booting with only one stick in slot A2.
    • Swapped both sticks individually.
    • Also tried slot A1.
      No change — system remains stuck on CPU LED.

  4. CMOS/BIOS

    Performed a full reset:

    1. Powered off.
    2. Unplugged PSU.
    3. Held power button for 10 seconds.
    4. Removed CMOS battery for 2 minutes.
    5. Reinstalled battery.
      No improvement.

  5. CPU check.

    • Removed CPU cooler.
    • Cleaned and reapplied thermal paste.
    • Removed CPU and inspected all pins (none bent or missing).
    • Reseated CPU correctly.
    • Reinstalled cooler evenly.
      Still getting solid CPU debug LED.

Current Situation

Despite:

  • Correct PSU connections.
  • A known-good CPU with intact pins.
  • RAM testing.
  • CMOS reset.
  • GPU tests.
  • The system previously working after the upgrade.

…the motherboard stays stuck on the CPU debug LED, with no USB power and no display output.

This suggests the CPU is not being initialized at all.

My Questions

  • Could this be a sign of a faulty EPS rail or port on the PSU?
  • Could the CPU_PWR1 port or VRM circuitry on the motherboard be damaged?
  • Is the next logical step to test with another PSU?
  • Any other diagnostics I should perform before replacing parts?

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