Start with the simplest test: does your computer turn on at all?

A dead power supply usually announces itself—your computer won't start, or it starts and shuts down within seconds. But a failing power supply can also cause random crashes, unexpected restarts, or components that work intermittently. Before you assume the power supply is the problem, you need to rule out simpler causes and then run a few straightforward tests.

The first thing to check is whether power is actually reaching your computer. Make sure the wall outlet works by plugging in a lamp or phone charger. Check that the power cable is fully seated in both the wall outlet and the back of your power supply. If your power supply has a physical on/off switch (usually a small rocker switch on the back), make sure it's in the ON position. These three steps solve the problem more often than you'd expect.

If your computer still won't start, move to the next test. If it does start but you suspect the power supply is failing, you'll want to measure the voltage it's actually producing.

Key Takeaways

  • A multimeter set to DC voltage is the standard tool for testing whether a power supply is delivering the correct voltage to your motherboard and components.
  • The 24-pin motherboard connector should read between 11.4 and 12.6 volts on the 12V rail and between 4.75 and 5.25 volts on the 5V rail when the computer is running.
  • The paperclip test lets you check whether a power supply can turn on by itself without a motherboard connected, which tells you if the problem is the supply or something else in the system.
  • A power supply that fails any voltage test, makes unusual noises, or smells like burning plastic should be replaced rather than repaired.
  • If your computer crashes randomly or restarts without warning, the power supply is one of several possible causes—check temperatures and hard drive health before replacing it.

Use a multimeter to measure the voltage your power supply is producing

A multimeter is an inexpensive tool (usually $15 to $30) that measures electrical voltage. You can find one at any hardware store or online. Set the dial to DC voltage (marked as DCV or V with a straight line above it, not a wavy line). The wavy line setting is for alternating current, which is not what you need.

With your computer powered on and running normally, locate the 24-pin connector that plugs into your motherboard. This is the largest connector coming from the power supply. You don't need to unplug it—you can test it while it's connected. Touch the black probe of the multimeter to any black wire (ground), and touch the red probe to the wire you want to test. The 12V rail (usually a yellow wire) should read between 11.4 and 12.6 volts. The 5V rail (usually a red wire) should read between 4.75 and 5.25 volts. If either reading is outside this range, your power supply is not delivering stable power.

You can also test the 8-pin or 4-pin CPU power connector (usually near the top of the motherboard) using the same method. The 12V rail there should also read 11.4 to 12.6 volts. If the readings are significantly lower or fluctuate wildly, the power supply is likely failing.

Try the paperclip test to see if the power supply can turn on by itself

The paperclip test tells you whether the power supply itself can power on, independent of your motherboard or any other component. This is useful because sometimes a computer won't start because of a bad motherboard, not a bad power supply.

First, unplug the power supply from the wall and from all components. Locate the 24-pin motherboard connector. Find the green wire and any black wire in that connector. Straighten a paperclip and insert one end into the connector slot next to the green wire, and the other end into a slot next to a black wire. Plug the power supply back into the wall outlet. The power supply fan should spin. If it does, the power supply itself is working. If the fan doesn't spin, the power supply is likely dead.

Once you've confirmed the fan spins, unplug the power supply again and remove the paperclip. This test doesn't tell you whether the voltage is correct—only that the supply can turn on. For a complete picture, use the multimeter test above.

Check for physical damage and unusual sounds or smells

Before you run electrical tests, do a visual inspection. Open your computer case and look at the power supply. Check for burn marks, melted plastic, or bulging capacitors (the cylindrical components inside the supply). If you see any of these, the power supply is damaged and should be replaced.

Listen to the power supply while your computer is running. A healthy power supply is nearly silent or makes a very quiet hum. If you hear a loud whining noise, clicking, or grinding sound, the power supply is failing. A burning smell coming from the power supply is a sign to shut down when ready and unplug it—do not run any more tests.

Understand what voltage readings actually mean

Power supplies are rated by their maximum wattage (like 650W or 750W), but what matters for testing is whether they're delivering the correct voltage on each rail. Modern power supplies have three main voltage rails: 12V, 5V, and 3.3V. Most of your components draw power from the 12V rail, which is why that's the most important one to test.

The tolerance ranges (11.4 to 12.6 volts for the 12V rail, for example) exist because voltage naturally fluctuates slightly. A reading of 12.0 volts is ideal. A reading of 11.5 volts is fine. A reading of 10.8 volts means the supply is struggling and may cause instability. A reading below 10.5 volts or above 13 volts means the power supply is out of spec and should be replaced.

If your readings are within spec but your computer is still crashing or restarting, the problem may not be the power supply. Random restarts can also be caused by overheating, a failing hard drive, or loose internal cables. Check your CPU and GPU temperatures using free software like HWiNFO or GPU-Z before you assume the power supply is at fault.

Know when to replace the power supply instead of troubleshooting further

Power supplies are not user-serviceable. If your multimeter shows out-of-spec voltage, if the paperclip test shows the fan won't spin, or if you see physical damage, the power supply needs to be replaced. Do not attempt to open it or repair it yourself—power supplies contain capacitors that can hold a dangerous charge even when unplugged.

When you buy a replacement, match the wattage of your old supply or go slightly higher. A 650W supply can be replaced with a 750W supply without any problem. Going lower (replacing a 750W with a 650W) can cause instability if your system draws close to the maximum. Look for supplies from established brands like Corsair, EVGA, Seasonic, or Thermaltake, which tend to be more reliable than generic no-name supplies.

Frequently Asked Questions

Can I test the power supply while it's still plugged into my motherboard?

Yes. The multimeter test works fine with everything connected and the computer running. The paperclip test requires you to unplug the power supply and disconnect it from all components first. Never touch the inside of the power supply while it's plugged in, even if the computer is off.

What if my multimeter readings are slightly low but the computer works fine?

Readings within the tolerance range (11.4 to 12.6 volts for 12V, 4.75 to 5.25 volts for 5V) are normal. If your readings are within spec and your computer is stable, the power supply is fine. If readings are consistently below 11.4 volts and your computer is crashing or restarting, the power supply is likely the cause.

My computer keeps restarting randomly. Is it definitely the power supply?

Not necessarily. Random restarts can be caused by overheating, a failing hard drive, loose RAM, or driver issues. Check your CPU temperature first using free software like HWiNFO. If temperatures are normal, run a hard drive diagnostic tool. Test the power supply only after ruling out these more common causes.

Do I need a special multimeter for computer power supplies?

No. Any basic digital multimeter that measures DC voltage will work. You don't need an expensive or specialized meter. A $20 multimeter from a hardware store is perfectly adequate for this test.

What should I do if the paperclip test shows the fan won't spin?

The power supply is not turning on, which means it's either dead or the on/off switch on the back is in the OFF position. Check the switch first. If it's already in the ON position, the power supply has failed and needs to be replaced.