It's frustrating — you just installed that shiny new power supply, and now your PC randomly shuts down under load. Maybe it runs fine for hours, then dies in a game. Or it reboots when you plug in a USB drive. Been there. Let's fix this.
The Fix (Do This First)
- Reseat ALL cables — both ends. Unplug the 24-pin motherboard cable, the 8-pin CPU cable, and any GPU power cables. Push them in firmly until you hear the click. This is the #1 cause.
- Check modular cables — if you're using modular cables from an old PSU, swap them for the ones that came with the new PSU. Mixing cables between different PSUs (even same brand) can short pins or deliver wrong voltage.
- Test with a different wall outlet — some PSUs are sensitive to dirty power. Try a different circuit or a UPS.
- Run a stress test — use OCCT or Prime95 to trigger the shutdown. If it happens within 30 seconds, you have a bad PSU or cable.
70% of the time, reseating the 24-pin and CPU cables fixes it. Another 20% is using wrong modular cables. The remaining 10% is a defective unit.
Why This Works
What's actually happening here is the power supply is delivering unstable voltage to the motherboard. The reason reseating helps is that a loose pin creates high resistance. High resistance means voltage drops under load. The motherboard's voltage regulator sees that drop and triggers a protection shutdown.
The reason modular cables matter: each PSU manufacturer (even different models from the same brand) uses different pinouts on the PSU side. If you plug a cable from a Seasonic unit into a Corsair PSU, you might send 12V down a pin that expects 3.3V. That wrecks components and causes shutdowns. Always use the cables that came with the PSU.
The wall outlet thing is real but rare. Some newer PSUs have active PFC (power factor correction) that doesn't like old wiring or shared circuits. If your PC shuts down when the fridge kicks on, that's the issue.
Less Common Variations
Shutdown only when GPU draws power
If the PC dies only in games or rendering, the GPU power cables are suspect. PCIe power cables (6+2 pin) look almost identical to CPU EPS 12V cables (4+4 pin). They're not the same. The EPS cable has four yellow wires and four black wires. The PCIe cable has three yellow and five black. Plugging the wrong one into your graphics card can cause shutdowns or even damage.
Shutdown after 30 minutes of idle
This is usually a motherboard BIOS setting — wake-on-LAN, USB selective suspend, or C-states. But with a new PSU, check the power good signal. Some PSUs have a delay before they send the power_ok signal to the motherboard. If it's too short (under 100ms), the motherboard thinks the power is bad and shuts down. Fix: update your BIOS to the latest version. Or try a different PSU.
Random reboots, not shutdowns
Reboots are different. That's the motherboard sensing a short and restarting. Check if a standoff screw is touching the back of the motherboard where it shouldn't. Or a loose case screw fell under the board.
Prevention for Next Time
- Buy from reliable brands — Seasonic, Corsair, EVGA, be quiet!, Super Flower. Cheap PSUs lie about their specs and protections.
- Keep modular cables labeled — put the PSU model number on a label on each cable bag. Do not throw away the original cables.
- Install the PSU with the fan facing down (if your case has a bottom vent). The PSU pulls cool air from outside the case, not hot internal air. This extends capacitor life.
- Use a single rail PSU for high-power GPUs. Multi-rail units can trip overcurrent protection when a GPU spikes current, causing random shutdowns. Most modern PSUs let you switch between single and multi-rail mode in the manual.
One more thing: if you swapped PSUs because the old one died, check the motherboard for blown capacitors — bulging tops or brown residue near the CPU socket. A bad motherboard can kill a new PSU too.