Test your power supply before you replace it

A failing power supply often gets blamed for problems it did not cause. Before you buy a replacement, you can run three straightforward tests at home using tools you probably already own. The first test tells you whether the power supply is turning on at all. The second measures the voltage it is actually delivering. The third simulates a full load to see if it holds steady under stress. Together, these tests will show you whether the power supply is the real problem or whether you should look elsewhere.

You do not need special equipment or a technician. A multimeter — a device that measures electrical current and voltage — costs between $15 and $40 and will last you years. A paperclip and a few minutes of your time complete the toolkit.

Key Takeaways

  • The paperclip test tells you whether a power supply is turning on at all by jumping two pins on the 24-pin motherboard connector without the motherboard plugged in.
  • A multimeter measures the actual voltage coming from each cable, and readings should stay within 5 percent of the label (12V should read 11.4 to 12.6V, for example).
  • A load test runs the power supply under simulated heavy use to see whether voltage sags or the fan shuts off unexpectedly.
  • If the power supply fails any test, it is almost certainly faulty and should be replaced; if it passes all three, the problem lies elsewhere in the computer.
  • Never open the power supply case itself — the capacitors inside hold a dangerous charge even when unplugged.

The paperclip test: checking whether the power supply turns on

This test tells you whether the power supply is actually receiving power and attempting to start. You will need a paperclip, a 24-pin ATX motherboard connector (the large flat connector that plugs into the motherboard), and nothing else plugged into the power supply.

Unplug the power supply from the wall outlet. Locate the 24-pin connector and look at the pins from the front. Find pin 16 (the green wire) and pin 15 (a black wire next to it). Straighten a paperclip and insert one end into pin 16 and the other into pin 15. Plug the power supply back in and flip the switch on the back to ON. The fan inside should spin when ready. If it does, the power supply is receiving power and the control circuit is working. If the fan does not spin, the power supply is not starting, and it is likely faulty.

Turn the switch off and remove the paperclip. This test takes 30 seconds and tells you whether to move forward with the voltage tests or to assume the power supply is dead.

Measuring voltage with a multimeter

A multimeter is a handheld device with a dial or digital display that measures voltage, current, and resistance. You will use it to check whether the power supply is delivering the correct voltage on each rail. Set the multimeter to DC voltage (usually marked with a V with a straight line above it, not a wavy line). The wavy line is for AC voltage, which is not what you need here.

With the power supply still off and the paperclip still in place, plug it in and turn it on again. The fan should spin. Now touch the red probe of the multimeter to the tip of a yellow wire (12V rail) and the black probe to the tip of a black wire (ground). The reading should be between 11.4 and 12.6 volts. Repeat this test on a red wire (5V rail) — it should read between 4.75 and 5.25 volts. Check an orange wire (3.3V rail) — it should read between 3.14 and 3.47 volts. If any reading falls outside these ranges, the power supply is not regulating voltage correctly and should be replaced.

You can test multiple connectors on the same power supply — 24-pin, 8-pin CPU, 6-pin or 8-pin PCIe, and SATA connectors all carry the same rails. If one connector reads wrong, the whole power supply is faulty. If all connectors read within range, move to the load test.

The load test: checking stability under stress

A power supply can deliver correct voltage at idle but fail when the computer is under heavy load. A load test simulates that stress without risking your actual hardware. The simplest method is to use a piece of software called Prime95 or MemTest86, which puts the CPU and RAM under maximum load while you monitor the power supply.

Keep the paperclip in place and the multimeter probes touching the yellow and black wires. Start Prime95 (set it to stress test mode) or MemTest86 and let it run for at least 10 minutes. Watch the multimeter reading. The voltage should stay within the same 5 percent range even as the load increases. If the voltage sags below the minimum (12V dropping below 11.4V, for example) or if the power supply fan shuts off unexpectedly, the power supply cannot handle the load and is faulty.

If the voltage holds steady and the fan keeps running throughout the test, your power supply is working correctly. Any problems you are experiencing in the computer are caused by something else — a failing hard drive, bad RAM, a loose cable, or a motherboard issue.

What to do if the power supply fails a test

If the paperclip test shows no fan spin, the power supply is not starting and needs to be replaced. If the voltage test shows readings outside the acceptable range, or if the load test shows voltage sag or fan shutdown, the power supply is faulty and should be replaced.

When you buy a replacement, match the wattage to your system. A graphics card, CPU, and storage drive all draw power, and the power supply must have enough headroom to handle them all at once. Most modern systems need between 550 and 750 watts. If you are unsure, add up the power draw of your largest components (usually listed on their boxes or in their manuals) and add 30 percent as a safety margin. Buy a power supply with at least that much capacity.

Why you should not open the power supply case

The inside of a power supply contains large capacitors that store electrical charge. Even when the power supply is unplugged from the wall, these capacitors can hold enough charge to cause a serious shock or burn. There is no user-serviceable part inside a power supply. If it fails any test, replace it rather than trying to repair it.

The three tests described here — paperclip, voltage, and load — tell you everything you need to know about whether the power supply is working. You do not need to open it or probe inside it.

Frequently Asked Questions

Can a power supply fail one test but pass another?

Yes. A power supply might spin its fan (passing the paperclip test) but deliver incorrect voltage (failing the voltage test). This means the control circuit is working but the regulation circuit is broken. Any failure on any test means the power supply should be replaced.

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

Most multimeters have a DC voltage setting — look for a V with a straight line above it on the dial. If yours does not, you have an older or very basic model. A new multimeter costs $15 to $25 and is worth the investment for future troubleshooting.

Do I need to unplug everything from the power supply before testing?

Yes. The paperclip test and voltage test should be run with nothing plugged into the power supply except the 24-pin motherboard connector. The load test can run with the motherboard and CPU plugged in, but not the graphics card or other high-draw components, since you are only simulating load with software.

How long should I run the load test?

At least 10 minutes. Most power supply failures show up within the first 15 minutes of heavy load. If the voltage stays stable and the fan keeps running for 15 minutes, the power supply is stable enough for normal use.

What if the voltage is slightly off but close to the acceptable range?

Stay within the 5 percent margin. A 12V rail reading 12.8V is outside the range and indicates a problem. A 12V rail reading 12.5V is within range and is normal. If you are unsure, write down the reading and compare it to the label on the power supply itself — the label will show the acceptable range for that specific model.