How to tell if your PSU is failing before it damages your computer

A failing power supply unit (PSU) usually shows itself through symptoms before it stops working entirely: your computer shuts down without warning, won't turn on at all, makes a high-pitched noise, or restarts randomly during normal use. The fastest way to know whether the PSU is the problem is to listen for the fan, watch for lights, and test whether the computer responds to the power button. If the PSU is completely dead, your computer won't start no matter what you do. If it's failing intermittently, the symptoms come and go.

You can do several checks yourself without opening the case or using special tools. Some require opening the computer, but the process is straightforward. If you find the PSU is the problem, replacing it is one of the most common repairs a computer owner can do.

Key Takeaways

  • A working PSU will have a fan that spins (usually visible through a vent on the back of the case) and will power on when you press the power button.
  • If your computer won't turn on at all, check that the power cable is plugged in, the outlet works, and the power supply switch on the back of the case is set to ON.
  • Random shutdowns, restart loops, and burning smells are signs the PSU may be failing and should be replaced soon.
  • You can test the PSU with a paperclip while it is unplugged to see if the fan spins, but this only tells you if power reaches the unit—not whether it is delivering correct power to your components.
  • If basic checks do not reveal the problem, the PSU may be failing in a way that requires replacement or professional diagnosis.

Check the power cable and outlet first

Before you assume the PSU is broken, verify that power is actually reaching it. Look at the back of your computer case. You should see a thick black cable plugged into the PSU and into a wall outlet or power strip. If the cable is loose or unplugged, plug it in firmly and try turning on the computer.

Test the outlet itself by plugging in a lamp or phone charger—something you know works. If the outlet is dead, try a different one. If the outlet works but your computer still won't turn on, move to the next check. Also look for a small rocker switch on the back of the PSU itself, near where the power cable enters. It should be set to the ON position (usually marked with a line, not a circle). If it is switched to OFF, flip it to ON and try again.

Listen and look for signs of power

Press the power button on the front of your case and listen carefully. A working PSU will have a fan inside it that spins when power flows through. You may hear a faint whirring sound, and you should see the fan blades moving if you look at the vent on the back of the case (usually a square opening with a fan visible inside). You may also see lights turn on inside the case or on the motherboard.

If you hear nothing, see no fan movement, and no lights come on, the PSU is not delivering power. If the fan spins for a moment and then stops, or if it spins but the computer does not boot, the PSU may be failing. If everything powers on but the computer shuts down after a few seconds or minutes, the PSU may be overheating or failing under load.

Watch for warning signs during normal use

Even if your computer turns on, the PSU can fail gradually. Random shutdowns in the middle of work—especially when you are using demanding programs like games or video editing—often point to a PSU that cannot deliver enough power under load. The computer may restart by itself, or it may shut down completely and refuse to turn back on for a few minutes.

A burning smell coming from the back of the case is a sign the PSU is overheating or a component inside is failing. If you smell this, unplug the computer when ready and do not use it until the PSU is replaced. A high-pitched whining or coil whine from the PSU is also a warning sign that it is stressed. Blue sparks or visible damage inside the PSU vent are signs of internal failure.

The paperclip test to check if the PSU fan spins

If your computer will not turn on and you want to know whether the PSU itself has power, you can do a straightforward test without opening the case fully. Unplug the computer from the wall outlet completely. Look at the 24-pin connector on the PSU—this is the large rectangular plug that connects the PSU to the motherboard. You do not need to remove it.

Find the green wire and any black wire in that connector. Straighten a paperclip and insert one end into the connector so it touches the green wire, and insert the other end so it touches a black wire next to it. Plug the computer back in (but do not press the power button). The PSU fan should spin. If it does, the PSU is receiving power and the fan works. If it does not spin, the PSU may be dead or the outlet is not working.

This test only tells you whether the PSU fan spins—it does not tell you whether the PSU is delivering the correct voltage to your components. A PSU can fail in ways that allow the fan to spin but prevent it from powering your computer properly. If the fan spins but your computer still will not start, the problem may be elsewhere, or the PSU may be failing in a more subtle way.

When to replace the PSU instead of troubleshooting further

If you have checked the power cable, the outlet, the power switch, and the PSU fan, and your computer still will not turn on or shuts down randomly, the PSU is likely the problem. PSUs are not repairable—they are replaced as a unit. A new PSU costs between $50 and $200 depending on the wattage and quality, and installation takes about 15 minutes if you are comfortable opening the case.

If you are not comfortable replacing it yourself, a computer repair shop can do it for you. Before you buy a replacement, note the wattage of your current PSU (printed on the label on the back of the case) and buy one with the same or higher wattage. If you are upgrading your graphics card or adding new components, you may need a higher-wattage PSU—a technician can advise you on this.

Frequently Asked Questions

Can a PSU fail partially and still power the computer?

Yes. A failing PSU may deliver power to some components but not others, or may deliver unstable voltage that causes random crashes. This is why a computer can turn on but shut down under load, or why it may boot but not stay on long enough to load the operating system.

Is the paperclip test safe?

The paperclip test is safe if you unplug the computer first and do not touch any other parts of the PSU or motherboard. The 24-pin connector carries power, but with the computer unplugged, there is no active current. Still, if you are uncomfortable doing this, skip it and rely on the other checks instead.

What wattage PSU do I need?

Check the label on your current PSU for the wattage rating. For a replacement, match that wattage or go higher. If you are unsure, a 650-watt PSU works for most desktop computers. If you have a high-end graphics card or are building a new system, you may need 750 watts or more—a technician can help you choose.

Why does my PSU fan spin but my computer won't start?

The PSU fan spinning means power is reaching the unit, but it does not mean the PSU is delivering correct voltage to the motherboard and other components. The problem could be a failed PSU, a loose cable inside the case, a failed motherboard, or a failed CPU. If the fan spins but nothing else happens, the PSU may still be the culprit, or you may need professional diagnosis.

Can I use a PSU from another computer?

Yes, if it has the same connector type and equal or higher wattage. Most modern computers use a 24-pin ATX connector. Check that the PSU fits in your case and that all the cables reach your components. If you are borrowing one to test, make sure to return it and buy your own replacement for permanent use.