How to tell if your power supply is actually delivering power
A power supply unit (PSU) converts wall electricity into the right voltage and current for each part of your computer. When it fails, your whole system stops — but the computer won't always tell you the PSU is the problem. The fastest way to know is to listen and look: a working PSU has a fan that spins, and your motherboard lights up the moment you flip the switch on the back. If nothing happens when you press the power button, or if the computer shuts down randomly under load, the PSU is often the culprit.
You can narrow down whether the PSU itself is broken without opening the case or using a multimeter. The checks below work on any desktop computer and take about five minutes.
Key Takeaways
- A working PSU's fan spins when you turn on the computer, and the motherboard lights up when ready — if neither happens, the PSU may be dead.
- Flip the physical switch on the back of the PSU to off, wait ten seconds, then back on — this resets the PSU and fixes many temporary failures.
- Listen for the PSU fan and watch for motherboard indicator lights when you press the power button; silence and darkness usually mean a PSU problem.
- Unplug the 24-pin motherboard cable from the PSU while the computer is off, wait five seconds, and plug it back in — this clears stuck power states.
- If the PSU fan spins but the computer won't boot, the PSU may be delivering power but not the right voltage, and you will need a replacement.
Check for the PSU fan spinning and motherboard lights
The simplest check is visual and auditory. Turn on your computer normally by pressing the power button. Within one second, you should hear a faint whirring sound from the back or bottom of the case — that is the PSU fan starting. At the same time, look at the motherboard: most modern boards have at least one LED light that turns on when power arrives. This light is usually green or white and stays on as long as the computer is plugged in and the PSU switch is on.
If you hear the fan spin and see the light, the PSU is delivering power to the motherboard. If you hear nothing and see no light, the PSU is not working. If the fan spins but the light does not appear, the motherboard itself may be damaged, but the PSU is doing its job.
The motherboard light location varies by model — check your motherboard manual or look for a small LED near the CPU socket or along the bottom edge of the board. If you cannot find it, the fan spin alone is a good sign: a PSU that is completely dead will not spin the fan at all.
Reset the PSU with the physical power switch
Many PSU failures are temporary. The unit can get stuck in a protective state and refuse to turn on again until it resets. This happens most often after a power outage, a sudden shutdown, or a short circuit somewhere in the system.
Locate the physical power switch on the back of the PSU — it is a small rocker or toggle switch, usually labeled with I and O. Flip it to the O (off) position. Wait ten seconds. Flip it back to I (on). Now press your computer's power button as normal. This straightforward reset clears the PSU's internal protection circuit and often restores power.
If the computer boots normally after this reset, the PSU was stuck but is not permanently damaged. If nothing changes, move to the next step.
Reseat the motherboard power cable
The 24-pin cable that connects the PSU to the motherboard can loosen over time, especially if the case was moved or the computer was serviced. A loose connection means the motherboard receives no power even though the PSU is working.
Turn off the computer and flip the PSU switch to off. Wait five seconds. Locate the large rectangular connector plugged into the motherboard — it has 24 small pins arranged in two rows and is usually near the CPU socket. Gently pull this cable straight out of the motherboard socket. Look at both the cable end and the socket for bent pins or corrosion; if you see either, the cable or motherboard may need replacement.
If the connector looks clean, push it back in firmly until you hear or feel a click. The connector should sit flush against the motherboard with no gaps. Turn the PSU switch back on and press the power button. If the computer now boots, the loose cable was the problem.
Check for power delivery under load
A PSU can spin its fan and light up the motherboard but still deliver the wrong voltage to components. This usually shows up as random shutdowns or restarts when the computer is under heavy use — gaming, video editing, or running multiple programs at once.
If your computer boots fine but shuts down after a few minutes of use, the PSU is likely failing under load. The motherboard and graphics card are demanding more power than the PSU can safely deliver, so the PSU's protection circuit cuts power to prevent damage.
You can test this by running a stress test — a program that pushes your CPU and GPU to maximum load. Free options include Prime95 (for CPU) or FurMark (for GPU). Run one of these for five to ten minutes. If the computer shuts down or restarts, the PSU is the problem. If it stays stable, the PSU is probably fine and the issue lies elsewhere.
What to do if the PSU is not working
If the PSU fan does not spin, the motherboard light does not turn on, and the reset and reseating steps did not help, the PSU is dead and needs replacement. Before you buy a new one, check the wattage of your current PSU — it is printed on a label on the side or back of the unit. Your replacement should be the same wattage or higher.
If you are not comfortable opening the case or swapping the PSU yourself, a computer repair shop can do it for you in about an hour. The cost of a replacement PSU ranges widely depending on wattage and brand, but a reliable unit for a typical desktop costs between $50 and $150.
If the PSU fan spins and the motherboard lights up but the computer still will not boot, the problem is not the PSU — it is likely the motherboard, CPU, or RAM. In that case, you will need to troubleshoot those components separately.
Frequently Asked Questions
Can a PSU be partially broken?
Yes. A PSU can deliver power to some components but not others, or deliver the correct voltage at idle but fail under load. This is why a computer might boot but shut down during gaming, or why one component works while another does not. Partial failures are common as a PSU ages.
What does it mean if the PSU fan is spinning but nothing else happens?
The PSU is delivering some power, but the motherboard is not receiving it or is not responding. Check that the 24-pin motherboard cable is fully seated. If it is, the motherboard or CPU may be damaged, not the PSU.
Is it safe to test the PSU myself?
The checks in this guide — listening for the fan, looking for lights, reseating cables — are safe. Do not open the PSU itself or touch the internal components. PSUs store electrical charge even when unplugged and can cause serious injury.
How long does a PSU usually last?
A quality PSU lasts five to ten years under normal use. Cheaper units may fail sooner. Heat, dust, and constant heavy load all shorten lifespan. If your PSU is more than seven years old and failing, replacement is usually the right choice.
What if my computer powers on but when ready shuts down?
This usually means the PSU is delivering power but something is wrong with the motherboard, CPU, or RAM — not the PSU itself. Try reseating the RAM and the CPU cooler, then test again. If the problem persists, the motherboard may be damaged.