Start with the paperclip test for a quick diagnosis

The fastest way to check whether your power supply is working at all is the paperclip test. This tells you whether the PSU can turn on and produce power, though it does not test whether it delivers the right voltage to your components.

Unplug the power supply from the wall and from your motherboard. Locate the 24-pin ATX connector — the large rectangular plug that connects to your motherboard. Look at the pins inside the connector. Find pin 16 (green wire) and pin 15 (black wire) next to it. Straighten a paperclip and insert one end into pin 16 and the other end into pin 15. Plug the PSU back into the wall outlet. The fan should spin. If it does not, the PSU is likely dead. If the fan spins, the PSU can at least power on.

This test is safe because you are only bridging two pins that the motherboard normally bridges when it sends the power-on signal. Stop the test by unplugging the PSU from the wall.

Key Takeaways

  • The paperclip test checks whether the PSU fan spins when powered on, which is the first sign of life but not a full diagnosis.
  • A multimeter set to DC voltage mode can measure the actual voltage on each rail — 12V, 5V, and 3.3V — to see whether they match the PSU's rated output.
  • Testing under load (with the computer running) is more reliable than testing at idle, because many PSU failures only show up when the power supply is actually working hard.
  • If voltages drift more than 5 percent from their rated values, the PSU is degrading and should be replaced before it damages your components.
  • Specialized PSU testers exist but are not necessary for basic diagnosis — a multimeter costs less and works just as well.

Use a multimeter to measure voltage on each rail

A multimeter is a handheld meter that measures voltage, current, and resistance. For PSU testing, you only need to measure voltage. A basic digital multimeter costs between $15 and $40 and is worth owning if you work on computers regularly.

Set the multimeter to DC voltage mode (usually marked with a V with a line and dots, or a V with an arrow). Plug the PSU into the wall but do not plug it into the motherboard yet. Use the paperclip trick to turn the PSU on. Touch the red probe to the center pin of a Molex connector (the four-pin power connector used by older drives) and the black probe to one of the black ground pins next to it. You should read approximately 12 volts. Move to the yellow wire on the same connector — you should read approximately 5 volts. Test a few different Molex connectors to see whether the readings are consistent.

For the 24-pin ATX connector, the voltage rails are color-coded: yellow wires carry 12V, red wires carry 5V, and orange wires carry 3.3V. Test each color group. Acceptable voltage is within 5 percent of the rated value — so 12V should be between 11.4V and 12.6V, 5V between 4.75V and 5.25V, and 3.3V between 3.14V and 3.47V.

Test the PSU while the computer is running for a real-world check

A PSU that passes the paperclip test and the multimeter test at idle may still fail under load. The most reliable test is to measure voltage while the computer is actually running and drawing power from the PSU.

Boot the computer normally. While it is running, carefully insert the multimeter probes into the back of the 24-pin ATX connector or into a Molex connector without disconnecting anything. Test the 12V, 5V, and 3.3V rails again. If the voltages are stable and within range, the PSU is likely healthy. If the voltages drop significantly when the computer is under load — especially if they drop below the acceptable range — the PSU is failing and should be replaced.

You can also run a stress test like Prime95 or MemTest86 while measuring voltage. These programs push the CPU and RAM hard, which draws maximum power from the PSU. If voltages hold steady during a stress test, the PSU is probably good. If they sag or fluctuate wildly, the PSU cannot handle the load.

Watch for warning signs that point to a failing PSU

Some problems are easier to spot by behavior than by testing. A PSU that is failing often shows one or more of these signs: the computer randomly shuts down or restarts without warning, especially during gaming or video editing when power demand is high. The computer fails to start on the first try but starts on the second or third attempt. You hear a buzzing or humming noise from the PSU that is louder than the normal fan noise. The PSU fan does not spin at all, even when the computer is under load.

If you see any of these signs, test the PSU with a multimeter under load. If the voltages are unstable or out of range, replace the PSU. Do not wait — a failing PSU can damage your motherboard, CPU, or storage drives if it delivers incorrect voltage.

Know when a PSU tester tool is worth buying

A dedicated PSU tester is a small device that plugs into the 24-pin ATX connector and displays the voltage on each rail without needing a multimeter. These cost between $20 and $60. They are convenient if you test PSUs often, but they are not necessary for occasional diagnosis.

A PSU tester shows voltage at idle only, not under load. For that reason, a multimeter is actually more useful — it lets you test while the computer is running, which catches more failures. If you own a multimeter already, you do not need a PSU tester. If you do not own either and you only test a PSU once every few years, buy a multimeter instead.

Replace the PSU if it fails any test

If the paperclip test shows the fan will not spin, the PSU is dead and must be replaced. If the multimeter shows voltages outside the acceptable range, or if voltages are unstable under load, the PSU is degrading and should be replaced before it fails completely and damages other components.

When you buy a replacement, match the wattage to your system. Add up the power draw of your CPU, GPU, and storage drives — most modern systems need between 550W and 850W. Buy a PSU rated for at least 20 percent more than your system's peak draw, because a PSU that runs at maximum capacity all the time fails faster than one with headroom. Look for 80 Plus Bronze certification or higher, which means the PSU is at least 80 percent efficient and will waste less power as heat.

Frequently Asked Questions

Can I test a PSU without unplugging it from the motherboard?

Yes. You can measure voltage on the 24-pin ATX connector or Molex connectors while they are plugged in. The paperclip test requires unplugging the 24-pin connector because you need to bridge two pins that the motherboard normally bridges. Testing with a multimeter while the computer is running is actually the most realistic test because it shows how the PSU behaves under real load.

What does it mean if the fan spins but the computer will not start?

The PSU can power on but may not be delivering the correct voltage to the motherboard. Use a multimeter to check the 12V, 5V, and 3.3V rails. If any voltage is out of range, the PSU is the problem. If all voltages are correct, the problem is likely the motherboard, CPU, or RAM, not the PSU.

How often should I test my PSU?

Test your PSU only if the computer shows signs of failure — random shutdowns, failure to start, or unusual noise from the power supply. A healthy PSU does not need testing. If you replace the PSU, keep the old one and test it to confirm it was actually bad before you throw it away.

Is it safe to touch the inside of the PSU while testing?

No. Never open the PSU case or touch the components inside. A PSU stores electrical charge even when unplugged, and touching the wrong part can cause serious injury or death. All the tests described here are done from outside the PSU case.

What if my multimeter does not have a DC voltage setting?

A basic digital multimeter always has a DC voltage setting — it is the most common feature. If your meter does not, it is not a real multimeter. Buy a new one. A basic model with DC voltage, AC voltage, and resistance settings costs $15 to $25 and is all you need.