Check your PSU with the paperclip test or by reading its label

The fastest way to know if your power supply unit (PSU) is working is the paperclip test: unplug the PSU, open its fan vent, insert a straightened paperclip into the green and black pins of the 24-pin connector, plug it back in, and watch whether the fan spins. If the fan turns, the PSU is receiving power and likely functional. If nothing happens, the PSU may be dead.

Before you do that test, check the PSU's label on the back or side. Write down the wattage (usually 500W, 650W, 750W, or higher) and the model number. This information matters because a PSU can spin its fan but still fail to deliver stable power to your components—and knowing the specs helps you understand what your PC actually needs.

If your PC won't start at all, the paperclip test tells you whether the problem is the PSU or something else. If the PSU fan spins during the test but your PC still won't boot, the fault is likely your motherboard, CPU, or RAM. If the fan doesn't spin, the PSU itself is the problem.

Key Takeaways

  • The paperclip test—inserting a paperclip into the 24-pin connector's green and black pins—shows whether your PSU can power on at all.
  • A spinning fan during the paperclip test means the PSU is receiving power, but does not may provide it is delivering stable voltage to your components.
  • If your PC won't start but the PSU fan spins in the paperclip test, the problem is likely your motherboard, CPU, or RAM, not the power supply.
  • You can also test your PSU by checking for bulging or leaking capacitors on the circuit board, which are visible signs of failure.
  • A multimeter can measure the actual voltage output from your PSU connectors, but this requires opening your case and knowing which pins to test.

The paperclip test step by step

Shut down your PC completely and unplug the power cable from the back of the PSU. Wait 30 seconds to let any remaining charge drain. This step is important—touching live components inside a PSU can cause serious injury.

Locate the 24-pin ATX power connector on the PSU. This is the largest connector coming out of the PSU; it plugs into the motherboard. Look at the pins inside the connector. You need to find the green pin and any black pin next to it. The green pin is typically in the middle of the connector, and black pins (ground) are scattered throughout.

Straighten a paperclip so it forms a single wire. Insert one end into the hole of the green pin and the other end into the hole of an adjacent black pin. The paperclip now bridges the connection that tells the PSU to turn on. Plug the power cable back into the PSU and flip the switch on the back of the PSU to the ON position.

Watch the PSU fan. If it spins for a few seconds, the PSU is receiving power and its basic circuitry is working. If the fan does not move at all, the PSU is not responding to the power signal—a sign of failure. After 10 to 15 seconds, unplug the power cable and remove the paperclip.

What a spinning fan actually tells you

A fan that spins during the paperclip test means the PSU can turn on, but it does not mean the PSU is delivering the correct voltage to your PC. A PSU can spin its fan while outputting unstable or incorrect power, which would cause your PC to crash, freeze, or fail to boot even though the PSU appears to be working.

If your PC won't start and the PSU fan spins in the paperclip test, the problem is somewhere else in your system. Check that the 24-pin connector and the 8-pin CPU power connector are both fully seated in the motherboard. Reseat your RAM by removing each stick and pushing it firmly back into its slot. These are the most common reasons a PC fails to boot when the PSU is functional.

If the fan does not spin at all during the paperclip test, the PSU is not responding to the power signal. This usually means the PSU has failed internally and needs to be replaced.

Visual inspection for capacitor damage

Before you run the paperclip test, open your PC case and look at the circuit board inside the PSU. You do not need to open the PSU itself—just look through the fan vent or any opening. Look for capacitors, which are small cylindrical components standing upright on the board. A healthy capacitor has a flat top. A failed capacitor will have a bulging or domed top, or you may see a dark liquid or residue leaking from the top.

Bulging or leaking capacitors are a clear sign that the PSU has failed and should be replaced. This is one of the most common failure modes in power supplies. If you see this damage, do not attempt to use the PSU—replace it.

If the capacitors look normal and flat, the PSU may still be functional, and the paperclip test will tell you more.

Using a multimeter to measure voltage

A multimeter is a tool that measures electrical voltage. If you own one and know how to use it, you can test the actual voltage output from your PSU connectors. This is more thorough than the paperclip test, but it requires opening your case and knowing which pins to test.

Set your multimeter to DC voltage mode. Locate the 24-pin ATX connector on the motherboard (or on a loose cable if you are testing the PSU outside the case). The red wire pins should output +12V, the yellow wire pins should output +12V, the red wire pins should output +5V, and the black wire pins are ground (0V). Touch the multimeter's red probe to a red wire pin and the black probe to a black wire pin. The reading should be close to +5V. Test a yellow wire pin against a black pin—it should read close to +12V.

If the readings are significantly off (more than 10% lower than they should be), the PSU is not delivering stable power and should be replaced. If the readings are within range, the PSU is likely functioning correctly.

When to replace your PSU

Replace your PSU if the paperclip test shows no fan activity, if you see bulging or leaking capacitors, or if a multimeter shows voltage readings that are too low. A failing PSU can damage other components in your PC, so it is better to replace it than to risk losing your motherboard or CPU.

When you buy a replacement, match or exceed the wattage of your old PSU. If your old PSU was 650W, buy a new one that is 650W or higher. Check the model of your motherboard and CPU to understand the power requirements—most modern gaming PCs need between 650W and 850W, while office PCs often need only 400W to 500W.

Installation is straightforward: unplug the old PSU, disconnect all its cables from the motherboard and drives, remove the four screws holding it to the case, slide it out, and slide the new one in. Reconnect all cables in the same positions they came from, and your PC should boot normally.

Frequently Asked Questions

Can a PSU fail if the fan is spinning?

Yes. A PSU can spin its fan while delivering unstable or incorrect voltage to your components. If your PC crashes, freezes, or won't boot even though the PSU fan spins, the PSU may be failing. A multimeter test or replacement with a known-good PSU is the way to confirm.

Is the paperclip test safe?

The paperclip test is safe if you unplug the PSU first and wait 30 seconds before inserting the paperclip. A PSU can store electrical charge even when unplugged, so waiting is important. Never touch the inside of the PSU with your bare hands while it is plugged in.

What wattage PSU do I need?

Check your motherboard and CPU specifications. Most office PCs need 400W to 500W. Gaming PCs with a mid-range graphics card need 650W to 750W. High-end gaming builds need 850W or more. Buy a PSU rated for at least your system's peak power draw, plus 20% headroom.

How long does a PSU usually last?

A quality PSU typically lasts 5 to 10 years with normal use. Cheaper units may fail sooner. Heat, dust, and power surges shorten lifespan. If your PSU is more than 7 years old and your PC is acting unstable, PSU failure is worth investigating.

Can I test the PSU while it is still connected to the motherboard?

Yes, the paperclip test works with the PSU still connected to the motherboard. The paperclip bridges the power-on signal, and the motherboard will not be damaged. However, if you are testing a PSU outside the case, make sure no metal parts of the PSU touch the case or any conductive surface while it is powered on.