Test your power supply without opening your computer first

The fastest way to know if your power supply is failing is the paperclip test: jump two pins on the motherboard connector with a paperclip while the supply is unplugged, then plug it in. If the fan spins, the supply itself works. If the fan does not spin or the computer will not start, the supply is likely dead. This takes five minutes and tells you whether to replace it or look elsewhere for the problem.

If the paperclip test passes but your computer still will not start, or if you want to measure the actual voltage the supply is sending, you will need a multimeter — a tool that costs $15 to $30 and reads electrical current. A multimeter tells you whether the supply is delivering the right voltage to each part of your computer, which narrows down whether the supply itself is the culprit or whether something else is drawing too much power and triggering a shutdown.

Key Takeaways

  • The paperclip test confirms whether your power supply fan will spin at all, which rules out a completely dead supply in under five minutes.
  • A multimeter measures the actual voltage output from each connector and costs $15 to $30; you do not need an expensive one for this job.
  • The 24-pin motherboard connector should read 12 volts on the yellow wires and 5 volts on the red wires when the supply is running.
  • If your computer shuts down under load but the paperclip test passes, the supply may be failing under stress rather than completely dead.
  • Swapping in a known-working supply from another computer is the most reliable way to confirm the problem if you do not own a multimeter.

The paperclip test: checking if the supply fan will turn on

Unplug your computer and the power supply from the wall. Open the case and locate the 24-pin connector that plugs into the motherboard — it is the largest rectangular connector coming from the power supply. Look at the pins on the back of the connector (the side that faces away from you when it is plugged in).

Find the green wire and the black wire next to it. The green wire is pin 16 on the connector. Insert a paperclip so that it touches both the green pin and one of the black pins beside it. Plug the power supply back into the wall outlet — do not plug the computer in yet. The fan inside the power supply should spin. If it does not, the supply is dead and needs replacement. If it does spin, the supply has power and the problem lies elsewhere.

Unplug the supply from the wall when ready after the fan starts. Remove the paperclip. This test only tells you whether the supply can turn on; it does not tell you whether it is delivering the right voltage to your components.

Using a multimeter to measure voltage on each connector

A multimeter is a handheld device with a dial and two probes — one red, one black. Set the dial to DC voltage (marked as V with a straight line, not a wavy one). The black probe always goes to a black wire (ground); the red probe touches the wire you are testing. You will test the 24-pin motherboard connector and the 8-pin CPU power connector, which is usually near the top of the motherboard.

With the computer plugged in and running (or with the paperclip test holding the supply on), touch the red probe to a yellow wire on the 24-pin connector and the black probe to a black wire on the same connector. The reading should be 12 volts, plus or minus 0.5 volts. Now test a red wire on the same connector — it should read 5 volts, plus or minus 0.25 volts. Test the orange wires; they should read 3.3 volts, plus or minus 0.15 volts.

If any reading is more than 0.5 volts off from the target, the supply is failing. If all readings are within range, move to the 8-pin CPU connector near the top of the motherboard. Test the yellow and black wires there — they should also read 12 volts. If the CPU connector reads low, the supply may be failing under the load of the processor.

Testing under load to catch supplies that fail when the computer works hard

Some power supplies pass the paperclip test and read correct voltage when idle, but fail when the computer is actually running and drawing power. If your computer shuts down during gaming, video editing, or other heavy tasks, but the multimeter readings look normal at rest, the supply is likely failing under load.

Run a stress test while monitoring voltages. Free tools like Prime95 or MemTest86 will push your CPU and RAM to maximum power draw. Start the stress test, then use the multimeter to check the 12-volt and 5-volt rails again. If the voltage drops more than 0.5 volts from the idle reading, the supply cannot handle the load and needs replacement. If voltage stays stable, the supply is probably fine and the shutdown is caused by overheating or a failing component elsewhere.

Swapping in a known-working supply to confirm the diagnosis

If you do not own a multimeter or want a faster answer, borrow a power supply from another desktop computer that you know works. Make sure it has the same connectors — a 24-pin motherboard connector and an 8-pin CPU connector, at minimum. Unplug your computer, disconnect the old supply, and plug in the borrowed one. Power on the computer.

If the computer starts and runs normally with the borrowed supply, your original supply is dead or failing. If the computer still will not start or shuts down under load, the problem is not the supply — it is the motherboard, CPU, RAM, or another component. This method is reliable but requires access to another working computer, which not everyone has.

When to replace the supply versus when to look elsewhere

Replace the power supply if the paperclip test shows no fan spin, if the multimeter readings are more than 0.5 volts off target, or if voltage drops significantly under load. A power supply that fails any of these tests will not reliably power your computer and will eventually damage other components.

Look elsewhere for the problem if the paperclip test passes, the multimeter readings are correct, and the computer runs fine under load. In that case, check whether your CPU or GPU is overheating, whether RAM is seated properly, or whether a hard drive is failing. A power supply that passes all tests is not the culprit.

What to look for when buying a replacement supply

Buy a supply with the same wattage or higher than your current one. Check your current supply's label — it will say something like "500W" or "750W". A higher wattage gives your components more headroom and makes the supply run cooler, which extends its life. Do not buy a supply with lower wattage than the one you are replacing.

Look for 80 Plus Bronze certification or better, which means the supply converts at least 85 percent of wall power into usable power for your computer. Supplies without this certification waste more energy as heat and fail sooner. Brands like Seasonic, Corsair, and EVGA are reliable; avoid no-name supplies, which often fail within a year.

Frequently Asked Questions

Can I test the power supply while it is still plugged into the motherboard?

Yes. The paperclip test works with the supply still connected to the motherboard. The multimeter test also works with everything plugged in — just make sure the computer is powered on or the paperclip is holding the supply on. Do not touch any metal parts of the computer while it is running.

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

The supply has power but may not be delivering enough voltage to the motherboard or CPU. Use a multimeter to check the 12-volt and 5-volt rails. If they read low, the supply is failing. If they read correct, the problem is the motherboard, CPU, or RAM, not the supply.

Is a multimeter safe to use on a running computer?

Yes, as long as you only touch the probes to the wires and connectors, not to any metal parts of the computer case or motherboard. The multimeter probes are insulated and draw almost no current. Never touch both probes to the same wire at the same time, and never touch a probe to ground and a live wire simultaneously with your bare hand.

How long does a power supply usually last?

Most supplies last five to ten years depending on how hard they work and how well they are cooled. A supply in a case with poor airflow or one that runs at maximum load every day will fail sooner. A supply in a well-ventilated case that runs at 50 percent load will last longer.

Can I fix a power supply myself, or do I have to replace it?

Power supplies contain high-voltage capacitors that can hold a charge even when unplugged, so opening one is dangerous. Replacement is safer and cheaper than repair. A new supply costs $50 to $150 depending on wattage; repair is not worth the risk.