The power supply unit (PSU) converts wall outlet electricity into the precise voltages your computer's components need — 12V for the processor and graphics card, 5V for USB and SATA devices, 3.3V for RAM and chipsets. When the power supply starts failing, the symptoms can mimic almost any other hardware problem: random shutdowns, blue screens, failure to boot, strange noises, and components not being detected. Because the PSU touches everything, a failing one can be hard to diagnose without knowing what to look for.
Symptoms of a failing power supply
Power supply problems usually show up in one of these patterns:
Computer does not turn on at all. You press the power button and nothing happens — no fans, no lights, no sounds. If the wall outlet works and the power cable is good, the PSU may have died completely
Computer turns on briefly then immediately shuts off. Fans spin for a half second to two seconds, then everything goes dead. This usually means the PSU is detecting a short circuit or overcurrent on one of its outputs and triggering its protection circuit
Random shutdowns under load. The computer works fine for light tasks but shuts down abruptly when you start a game, render a video, or run a stress test. Under heavy load, the CPU and GPU draw maximum power — a PSU that can no longer deliver its rated wattage fails at this point. There is no blue screen or shutdown sequence; the computer just cuts off as if someone pulled the plug
Random restarts. Similar to shutdowns but the computer restarts itself. This can also be caused by overheating or driver issues, but if it happens during high-power tasks and there is no blue screen error code, the PSU is a prime suspect
Blue screens with different error codes. A PSU delivering unstable voltage causes random memory errors, storage errors, and CPU faults — each producing a different BSOD error code. If you are getting blue screens with varying stop codes (not the same one every time), the power supply is worth investigating
Buzzing, clicking, or whining from the PSU. A faint electrical hum is normal (coil whine under load), but loud buzzing, grinding, or rhythmic clicking from the PSU area is not. Clicking may indicate a fan bearing failure; loud buzzing may indicate failing capacitors
Burning smell. A burning or acrid electrical smell from the back of the computer where the PSU sits is an emergency sign. Power off immediately, unplug, and do not power on again until the PSU is replaced. A burnt PSU can damage other components if it continues to operate
Basic checks before testing the PSU
Before opening the case, rule out the simple causes:
Power cable. Try a different power cable (the thick cable from the wall outlet to the back of the PSU). These cables are standard and interchangeable. A damaged cable can cause intermittent power loss
Wall outlet. Plug something else into the same outlet to confirm it works. Try a different outlet — ideally one on a different circuit. Avoid power strips with multiple high-draw devices
PSU switch. Check that the rocker switch on the back of the PSU (next to the power cable socket) is in the ON position (usually marked I). It can get bumped during cleaning or when moving the computer
Surge protector. If the PSU died during a storm or power flicker, the surge protector may have tripped. Reset it or try plugging the computer directly into the wall
The paperclip test
The paperclip test checks whether the PSU can turn on at all, independent of the motherboard. This tells you if the PSU is completely dead or if the problem is elsewhere:
Unplug the computer from the wall
Disconnect the 24-pin ATX cable from the motherboard (the large rectangular plug)
Find the green wire. The 24-pin connector has one green wire (PS_ON signal). On most PSUs, this is pin 16. It is usually the only green wire in the connector
Bridge the green wire to any black wire (ground) using a paperclip bent into a U shape or a short piece of wire. Insert one end into the green wire's pin hole and the other into an adjacent black wire's pin hole
Plug the PSU back into the wall and flip the rocker switch on
The PSU fan should spin. If the fan spins, the PSU can power on. If nothing happens, the PSU is dead
Important: the paperclip test only confirms the PSU can turn on — it does not test whether it delivers stable voltage under load. A PSU can pass this test but still fail when the computer's components draw real power. For a load test, you need a multimeter or a PSU tester.
Testing with a multimeter
A multimeter lets you check whether the PSU is delivering correct voltages on each rail. This is the most reliable way to diagnose a weak or failing PSU without specialized equipment:
Reconnect the PSU to the motherboard and power it on normally
Set the multimeter to DC voltage
Test the 24-pin connector while the system is running (carefully back-probe the pins or use a spare Molex/SATA connector):
Wire color
Expected voltage
Acceptable range
Yellow
+12V
11.4V to 12.6V
Red
+5V
4.75V to 5.25V
Orange
+3.3V
3.14V to 3.47V
Any reading outside those ranges indicates the PSU is failing. The +12V rail is the most important — it powers the CPU and GPU. If the +12V rail reads below 11.4V under load, the PSU cannot deliver adequate power.
Using a PSU tester
A dedicated PSU tester (available for $15 to $30) makes testing easier. You plug the 24-pin, 8-pin CPU, 6+2-pin GPU, and SATA connectors into the tester and it displays the voltage on each rail, flags out-of-range readings, and checks the Power Good signal. These are more convenient than a multimeter but less precise — a multimeter catches marginal readings that a basic tester may round off as passing.
Signs the PSU needs to be replaced
Failed paperclip test — the PSU does not turn on at all
Voltages out of range on any rail, especially under load
Age. PSUs degrade over time as their capacitors dry out. A PSU that is 7 to 10 years old, even if it technically works, may be delivering less stable power than it did when new. If you are troubleshooting intermittent problems on an older system, the PSU's age alone makes it a reasonable suspect
Physical signs: bulging or leaking capacitors visible through the PSU's fan grille, discoloration on the PCB, burn marks, or a persistent electrical smell
Insufficient wattage. If you upgraded your graphics card to a more powerful model without upgrading the PSU, the total power draw may now exceed what the PSU can deliver. Check the GPU manufacturer's recommended PSU wattage — this accounts for the whole system, not just the GPU
Choosing a replacement PSU
Wattage: add up your components' power draw and add 20 to 30 percent headroom. Most gaming PCs need 550 to 750W. Office and general-use desktops typically need 300 to 500W. Online PSU calculators (like the ones from Seasonic or Cooler Master) estimate based on your specific components
80 Plus rating: this certifies energy efficiency (how much wall power is wasted as heat). 80 Plus Bronze is the minimum worth buying. Gold and Platinum are more efficient and typically indicate better build quality
Modular vs non-modular: modular PSUs let you connect only the cables you need, which reduces clutter and improves airflow. Semi-modular (motherboard and CPU cables fixed, everything else detachable) is a good middle ground
Physical size: most desktop cases use ATX power supplies. Small form factor (SFF) and some prebuilt computers use SFX or proprietary sizes — measure before buying
PSU diagnosis and replacement in Marion
LuxuriousComputers Repair tests power supplies with calibrated equipment under real load conditions. We identify whether the PSU is the actual problem (not a symptom of another issue like a short on the motherboard), recommend a replacement matched to your system's needs, and install it with proper cable management. Same-day turnaround for most PSU replacements.