The Problem
The system powered off without warning during normal use. After that, pressing the power button caused fans and lights to flicker on for about one second before immediately shutting off again. No POST, no video output, no BIOS screen.
System Specs
- CPU: Intel Core i3-12100F
- Motherboard: Gigabyte B660M DS3H AX DDR4
- GPU: AMD Radeon RX 6650 XT
- RAM: DDR4 (2 sticks)
- Storage: NVMe M.2 SSD
- PSU: 550W ATX
Troubleshooting process
The approach was to eliminate components one at a time — start with the easiest swaps and work toward the hardest. Every step was documented before moving to the next.
Removed the RX 6650 XT from the PCIe slot, cleaned the contacts, and reseated it firmly. Powered on — behavior unchanged. One second of power, then shutdown. GPU ruled out as the cause.
Removed and reseated both sticks individually, then tested with one stick at a time in each slot. No change in behavior across all configurations. RAM ruled out.
Removed the 8-pin CPU power connector from the motherboard and powered on. The system behaved differently — it stayed powered slightly longer before shutting off. This was a key finding: the failure was happening during CPU power initialization, not before it.
With the CPU removed from the socket, powered on the system. The motherboard fans and lights stayed on normally and continuously. No immediate shutdown. This confirmed the shutdown was triggered by the motherboard's attempt to initialize the CPU power delivery circuit — not by the CPU itself.
With GPU, RAM, and storage all ruled out, and the failure pattern pointing specifically to the CPU power circuit, performed a part-swap test to confirm the motherboard was the faulty component. Confirmed — the B660M DS3H was the failed part.
Diagnostic logic
# Symptom Power on → 1 second → immediate shutdown No POST, no video, no BIOS # Step 1: GPU reseat Result: no change → GPU not the cause # Step 2: RAM isolation (each stick, each slot) Result: no change → RAM not the cause # Step 3: Remove CPU power connector (8-pin) Result: system stayed on longer → failure tied to CPU power init # Step 4: Remove CPU from socket Result: fans and lights stayed on normally → CPU not the cause system only fails when CPU power circuit is engaged # Conclusion Root cause: faulty motherboard (B660M DS3H AX) CPU power delivery / initialization circuit failed Resolution: motherboard replaced → system fully operational
Root cause & resolution
The Gigabyte B660M DS3H AX's CPU power delivery or initialization circuit had failed. The board was replaced and the system returned to full operation immediately, confirming the diagnosis.
When a system shuts down instantly with a CPU installed but runs normally without one, the motherboard is almost always at fault. The CPU doesn't cause shutdowns by existing in the socket — the board's power regulation circuit does.
What I learned
A no-POST could be RAM, GPU, PSU, CPU, or motherboard. Replacing parts randomly is expensive and slow. Systematic isolation identified the exact failed component without replacing anything unnecessarily.
"One second then off" is a specific symptom. It means the board is starting initialization and hitting a failure — not that it can't power on at all. That distinction narrowed the diagnosis significantly.
Removing the CPU to test the board in isolation is a standard hardware debug step. If the board runs without the CPU, the problem is in how the board handles CPU power — not the CPU itself.
Writing down each test and its result before moving on meant I always knew exactly what had been ruled out and what hadn't. No guessing, no repeated steps.