Testing a 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 quick tests at home with tools you likely already own: a multimeter, a paperclip, and your computer's power cable. These tests tell you whether the power supply is actually dead or whether the real problem is elsewhere—a motherboard fault, a loose cable, or a component drawing too much current.

The tests take about 15 minutes total and require no special skills. You will be checking whether the power supply produces the correct voltage on its cables, whether it powers on when it should, and whether it stays powered under load. If all three pass, your power supply is probably fine and you should look elsewhere for the problem.

Key Takeaways

  • A multimeter set to DC voltage mode can measure whether your power supply outputs the correct voltages on its cables without opening your computer.
  • The paperclip test forces a power supply to turn on outside the computer so you can hear and feel whether it is working at all.
  • A power supply that passes the paperclip test but fails under load in your computer may be failing and should be replaced.
  • Most power supplies output 12 volts on the yellow wires, 5 volts on the red wires, and 3.3 volts on the orange wires, with a tolerance of about 10 percent in either direction.
  • If your power supply passes all tests but your computer still will not start, the problem is likely the motherboard, CPU, or RAM, not the power supply.

What You Need to Perform the Tests

Gather a multimeter (a device that measures voltage, current, and resistance—available at any hardware store for $10 to $30), a paperclip, and your computer's power cable. You do not need to open your computer case for the first two tests. For the third test, you will need access to the power connectors inside your case, but you do not need to remove the power supply itself.

Set your multimeter to DC voltage mode before you start. The dial should point to a setting marked "DCV" or "V—" (the minus sign indicates direct current). If your multimeter has multiple voltage ranges, start with the 20-volt range; it will give you a clear reading for the voltages you are measuring.

The Paperclip Test: Checking If the Power Supply Turns On

This test forces the power supply to turn on without the computer's motherboard. Unplug the power cable from the wall, then locate the 24-pin connector that plugs into the motherboard. Look for the green wire and any black wire in that connector. Straighten a paperclip and insert one end into the socket that holds the green wire and the other end into a socket that holds a black wire next to it.

Plug the power cable back into the wall outlet. The power supply fan should spin when ready. If it does not spin at all, or if it spins for a second and stops, the power supply is not working and needs replacement. If the fan spins steadily, move to the voltage test.

Leave the paperclip in place for the next test. Do not touch the metal parts of the connector while the power supply is plugged in.

The Voltage Test: Measuring Output on Each Cable

With the power supply still running (paperclip still in place), use your multimeter to measure the voltage on the cables. You are checking three main voltages: 12 volts (yellow wires), 5 volts (red wires), and 3.3 volts (orange wires). Each voltage should stay within 10 percent of its target—so 12 volts should read between 10.8 and 13.2 volts, 5 volts between 4.5 and 5.5 volts, and 3.3 volts between 3.0 and 3.6 volts.

Touch the multimeter's red probe to a yellow wire and the black probe to any black wire (ground). The reading should be close to 12 volts. Repeat this for a red wire (should read close to 5 volts) and an orange wire (should read close to 3.3 volts). If all three readings fall within their ranges, the power supply is producing the correct voltages.

If any reading is far outside its range—for example, 12 volts reads as 8 volts or 15 volts—the power supply is failing and should be replaced. If the readings are correct but your computer still will not start, unplug the power cable, remove the paperclip, and move to the load test.

The Load Test: Checking Power Supply Stability Under Real Use

This test runs your power supply while your computer is actually running, so you can see whether it holds steady voltage when components are drawing power. Unplug the power cable, remove the paperclip, and reconnect the 24-pin motherboard connector and all other power cables to their proper places inside your case.

Start your computer normally. Once it has booted into Windows or your operating system, open a program that stresses the CPU and GPU—common choices are Prime95 (free, runs CPU stress tests) or FurMark (free, runs GPU stress tests). Let it run for 10 to 15 minutes while you monitor the voltages using software like HWiNFO or CPU-Z, both free downloads.

Watch the voltage readings for the 12-volt, 5-volt, and 3.3-volt rails. They should stay within the same 10 percent tolerance even while the stress test is running. If voltages drop significantly—for example, 12 volts drops to 10 volts or lower—the power supply cannot deliver enough current under load and is failing. If voltages remain stable, your power supply is working correctly.

What to Do If Tests Fail

If the paperclip test shows the fan does not spin, the power supply is dead and needs replacement. If the voltage test shows readings far outside the normal range, the power supply is failing. If the load test shows voltages dropping significantly under stress, the power supply is also failing and should be replaced.

When you buy a replacement, match the wattage of your current power supply or go slightly higher. Check your current power supply's label for its wattage rating (usually printed near the fan). A replacement should be at least the same wattage; going 50 to 100 watts higher provides a safety margin and is not wasteful.

If all three tests pass but your computer still will not start or crashes randomly, the problem is not the power supply. Look instead at the motherboard, RAM, CPU, or storage drive. A failing motherboard or bad RAM can cause symptoms that feel like a power problem but are not.

Common Mistakes to Avoid

Do not touch the metal parts of connectors while the power supply is plugged in, even during the paperclip test. The 12-volt and 5-volt rails can deliver enough current to cause a shock. Always unplug the power cable before inserting or removing the paperclip.

Do not assume the power supply is bad just because your computer will not start. Many people replace a working power supply only to find the real problem was a loose RAM stick or a dead motherboard. Run all three tests before you buy a replacement.

Do not run the stress test for more than 20 minutes on the first try. If the power supply is severely failing, prolonged stress can damage it further or cause a fire. If voltages drop when ready when you start the stress test, stop it and unplug the computer.

Frequently Asked Questions

Can I test the power supply without opening my computer case?

Yes, the paperclip test and voltage test both work on the external connectors. You only need to open the case if you want to measure voltages on cables inside the computer or run a load test while monitoring real-time voltage readings.

What if the fan spins but the voltage readings are zero?

The fan spinning means the power supply is receiving power, but zero voltage readings mean the output cables are not delivering power to the motherboard. This usually indicates a failed power supply. Try the voltage test on a different cable to confirm, then replace the power supply.

Do I need an expensive multimeter for this test?

No. A basic multimeter that measures DC voltage costs $10 to $20 and works perfectly for this test. You do not need a fancy model with extra features. Any multimeter with a DCV setting will do the job.

What if my power supply passes all tests but my computer still crashes?

Crashes under load can also be caused by a failing CPU, bad RAM, or an overheating GPU. Run a memory test using Windows Memory Diagnostic or Memtest86 to rule out RAM. Check CPU and GPU temperatures using HWiNFO. If those are normal, the motherboard itself may be failing.

Can a power supply fail partially and still pass these tests?

Yes. A power supply can fail on one rail while others still work, or it can fail only under very high loads. If your computer crashes only when running demanding games or software, but passes the load test, the power supply may be borderline. Consider replacing it as a precaution, especially if it is more than five years old.