Testing a Power Supply Without Opening Your Computer

You can test whether your power supply is working without taking apart your computer or using special equipment. The simplest test is the paperclip test: unplug the power supply from the wall, unplug all cables from inside the computer, then use a paperclip to bridge two specific pins on the 24-pin motherboard connector. Plug the power supply back in. If the fan spins, the power supply is likely working. If nothing happens, the power supply is probably dead.

This test works because bridging those two pins tricks the power supply into thinking it is connected to a motherboard and tells it to turn on. The fan spinning means the internal circuits are functioning. However, a spinning fan does not may provide the power supply is delivering the correct voltage to your computer—it only confirms the unit itself is not completely failed.

Key Takeaways

  • The paperclip test bridges two pins on the 24-pin connector to make the power supply turn on without a motherboard attached, and a spinning fan indicates the unit is not completely dead.
  • A multimeter can measure the actual voltage output on each cable, which tells you whether the power supply is delivering the correct power even if the fan spins.
  • If your computer will not start but the power supply fan spins during the paperclip test, the problem is likely the motherboard or another component, not the power supply.
  • Power supplies fail gradually—low voltage output can cause random crashes, freezes, or unexpected shutdowns before the unit stops working entirely.
  • Replacing a power supply is cheaper and faster than diagnosing individual components, so if the paperclip test passes but your computer still will not start, move on to testing other parts.

Locating the Right Pins for the Paperclip Test

The 24-pin motherboard connector has two rows of pins. You need to bridge pin 16 (the green wire) and any pin with a black wire next to it (ground). Pin 16 is in the bottom row, fourth pin from the left when you are looking at the connector from the front. The black ground pins are straightforward to spot—they are scattered throughout both rows.

If your power supply uses a 20-pin connector instead of 24-pin (older computers), the same principle applies: bridge the green wire to a black wire. The exact position varies slightly, so look for the green wire and find a black wire near it. Straighten a paperclip so it forms a bridge, insert one end into the green wire's pin slot and the other end into a black wire's pin slot, then plug in the power supply.

Using a Multimeter to Check Voltage Output

A multimeter is a handheld device that measures electrical voltage, and it costs between $15 and $50 at any hardware store. Set the dial to DC voltage (usually marked with a V and a straight line with dots underneath). Touch the red probe to the tip of a wire coming out of the power supply and the black probe to a black ground wire. The multimeter will display the voltage.

Each colored wire should output a specific voltage: red wires should read 5 volts, yellow wires should read 12 volts, and orange wires should read 3.3 volts. If any reading is more than 0.5 volts off (so red reads below 4.5 or above 5.5), the power supply is failing. Test at least two wires of each color to confirm the problem is not a single bad wire. If all readings are within range, the power supply is delivering power correctly.

What to Do If the Fan Does Not Spin

If the paperclip test produces no fan movement, the power supply is almost certainly dead and needs replacement. Before you buy a new one, confirm the power outlet itself is working by plugging in a lamp or phone charger. If that device works, the outlet is fine and the power supply is the problem.

Do not attempt to repair a power supply yourself. They contain capacitors that can hold a dangerous electrical charge even after being unplugged, and the internal components are not user-serviceable. Buy a replacement power supply with the same wattage as the original (check the label on the side of the old unit) and the same connector type (24-pin or 20-pin motherboard connector).

When the Fan Spins but Your Computer Still Will Not Start

If the paperclip test shows the fan spinning but your computer refuses to turn on, the power supply itself is probably not the problem. The issue is more likely a bad motherboard, a failed CPU, or a loose cable connection inside the case. Start by opening the case and reseating all power cables—unplug each one and plug it back in firmly until you hear a click.

Check that the 24-pin motherboard connector and the 8-pin CPU power connector (usually near the top of the motherboard) are both fully inserted. If reseating the cables does not help and the multimeter shows correct voltage on all wires, move on to testing the motherboard or other components. The power supply has passed both tests and is not your problem.

Signs Your Power Supply Is Failing Gradually

Power supplies do not always fail suddenly. A failing unit may deliver slightly low voltage, which causes your computer to crash randomly, freeze during heavy use, or shut down unexpectedly. You might see these symptoms for weeks before the power supply stops working entirely. If your computer is stable during light tasks but crashes when you play games or run video editing software, low power supply voltage is a common culprit.

The paperclip test will not catch this problem—the fan will still spin. Only a multimeter test will reveal that voltage is drifting below the safe range. If you see random crashes and the multimeter shows voltage readings that are consistently low (more than 0.5 volts below the target), replace the power supply. Do not wait for it to fail completely, because a failing power supply can damage other components as it dies.

Choosing a Replacement Power Supply

When you buy a new power supply, match the wattage of the old one. The label on the side of your current power supply shows the wattage—typically 450W, 550W, 650W, or higher. Buy the same wattage or slightly higher. Do not buy a lower wattage, because your computer may not have enough power to run all its components.

Also match the connector type: 24-pin motherboard connector for modern computers, 20-pin for older ones. Most power supplies sold today are 24-pin. Check your motherboard before you order to be certain. Installation is straightforward—unplug the old power supply, remove the four screws holding it to the case, disconnect all cables, then reverse the process with the new unit. Make sure all cables click firmly into place before you close the case and power on.

Frequently Asked Questions

Can a power supply fail if the fan is spinning?

Yes. A spinning fan means the power supply is not completely dead, but it can still deliver incorrect voltage to your computer. Use a multimeter to check the actual voltage output on the cables. If readings are more than 0.5 volts off target, the power supply is failing even though the fan works.

Is the paperclip test safe?

Yes, the paperclip test is safe. You are bridging two pins that the motherboard would normally bridge, so you are not creating any unusual electrical condition. Always unplug the power supply from the wall before inserting the paperclip, and remove it before plugging the power supply back into your computer.

What wattage power supply do I need?

Check the label on your current power supply—it shows the wattage. Buy a replacement with the same wattage or slightly higher. If you are upgrading to a more powerful graphics card or adding more storage drives, you may need a higher wattage, but for a direct replacement, match the original.

How long does a power supply usually last?

Most power supplies last five to ten years under normal use. Higher-quality units last longer. Heat and dust shorten lifespan, so keeping your computer clean and well-ventilated helps. If your power supply is more than seven years old and showing signs of failure, replacement is usually the best option.

Can I test the power supply while it is still connected to my computer?

The paperclip test requires the power supply to be disconnected from the motherboard, so you must unplug all cables first. A multimeter test can be done with the power supply connected, but only if you are very careful not to touch any metal parts inside the case while the power is on. Disconnecting everything is safer and takes only a few minutes.