Start with the simplest checks before you test anything

A dead or failing power supply usually shows itself in obvious ways first: your PC won't turn on at all, it shuts down randomly during use, or it powers on for a few seconds then cuts out. Before you test the power supply itself, rule out the easier culprits — a loose power cable, a tripped surge protector, or a wall outlet that has stopped working.

Plug a lamp or phone charger into the same outlet your PC uses. If that device works, the outlet is fine. If it doesn't, try a different outlet in a different room. If your PC still won't power on after you've confirmed the outlet works and the cable is seated firmly in both the power supply and the wall, then the power supply itself is the likely problem.

Key Takeaways

  • The paperclip test — jumping the green and black wires on the 24-pin connector — tells you whether the power supply can turn on at all, without risking your motherboard.
  • A working power supply's fan should spin when you do the paperclip test; no spin usually means the unit is dead.
  • A multimeter set to DC voltage can measure whether each rail is outputting the correct voltage, but only if you know what each wire color means.
  • If the power supply turns on during the paperclip test but your PC still won't boot, the problem is likely the motherboard or another component, not the power supply.
  • Power supplies fail gradually sometimes — random shutdowns or PC restarts during heavy use often point to a failing unit that still passes basic tests.

The paperclip test: checking whether the power supply can turn on

The paperclip test is the fastest way to know if your power supply is completely dead. It works by tricking the power supply into thinking it's connected to a motherboard, so you can see whether it powers up without risking damage to your actual PC components.

Unplug the power supply from the wall and from your PC. Locate the 24-pin connector — the large rectangular plug that connects the power supply to the motherboard. Look at the pins inside the connector. Find the green wire and any black wire next to it. Straighten a paperclip and insert one end into the socket holding the green wire and the other end into the socket holding the black wire. Plug the power supply back into the wall outlet only — do not plug it into the PC yet.

The power supply should turn on. You should hear the fan spin and see it rotate. If nothing happens, the power supply is not functioning and needs replacement. If the fan spins, remove the paperclip, unplug the power supply, and move to the next test.

Using a multimeter to check voltage on each rail

A multimeter is a handheld device that measures electrical voltage, current, and resistance. You can buy one for $15 to $30 at any hardware store or online. Set it to DC voltage mode (usually marked with a V with a line above it) and use it to measure whether each power rail inside your PC is outputting the correct voltage.

Power supplies have several different voltage rails, each marked by wire color. The yellow wires carry 12 volts, the red wires carry 5 volts, and the orange wires carry 3.3 volts. Black wires are ground (0 volts) and serve as the reference point for all measurements. Unplug the power supply from the wall, then plug it back in and turn it on using the paperclip test. Touch the multimeter's black probe to any black wire and the red probe to a yellow wire. The reading should be between 11.4 and 12.6 volts. Test a red wire — it should read between 4.75 and 5.25 volts. Test an orange wire — it should read between 3.15 and 3.45 volts.

If all three readings fall within those ranges, the power supply is outputting correct voltage. If any reading is far outside the range — for example, 12 volts reading as 8 volts — the power supply is failing and should be replaced. If the readings are correct but your PC still won't boot, the problem lies elsewhere in your system.

What to do if the power supply passes basic tests but your PC still won't start

Sometimes a power supply can pass the paperclip test and show correct voltage on a multimeter but still fail to power your PC. This usually means the problem is not the power supply itself but the motherboard, a loose internal cable, or a component that's preventing the system from booting.

Check that the 24-pin connector and the 8-pin CPU power connector (located near the top of the motherboard, close to the processor) are both seated firmly. Push them in until you hear or feel a click. Reseat your RAM by removing each stick and reinserting it firmly into its slot. Check that no cables are pinched or damaged. If you've done all this and the PC still won't start, the motherboard or CPU is likely the problem, not the power supply.

Signs of a power supply that's failing slowly

Not all power supply failures are sudden. Some units degrade over months or years, and the symptoms can be confusing because they mimic other hardware problems. If your PC randomly restarts or shuts down during heavy use — especially during gaming or video rendering — but runs fine during light tasks, a failing power supply is a common cause.

Random restarts happen because the power supply can't deliver enough current when all components draw power at once. The system detects the voltage drop and cuts power to protect itself. If you see this pattern, run the paperclip test and voltage checks. If the readings are borderline — for example, 12 volts reading as 11.0 or lower — the power supply is weakening and should be replaced soon, even if it hasn't failed completely yet.

When to replace the power supply versus when to keep testing

Replace the power supply when ready if the paperclip test shows no fan spin, if voltage readings are far outside the correct ranges, or if you see visible damage like burn marks, bulging capacitors, or a burnt smell coming from the unit. These are signs of failure that won't improve.

Keep the power supply if the paperclip test works, voltage readings are correct, and your PC boots normally. If your PC won't boot but the power supply passes all tests, the issue is almost certainly your motherboard, RAM, or another component. If you're not confident in your testing, a local computer repair shop can test the power supply for you, usually for $20 to $50, which is cheaper than replacing a unit that might still be working.

Frequently Asked Questions

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

No. The paperclip test must be done with the power supply unplugged from the motherboard. Testing while connected risks damaging the motherboard if something goes wrong. Always disconnect the 24-pin and 8-pin connectors before running the paperclip test.

What if the fan spins during the paperclip test but very slowly?

A slow fan spin usually means the power supply is failing. A healthy power supply fan should spin at a noticeable speed. If it's barely moving, use the multimeter to check voltage. If voltage is low or unstable, the unit should be replaced.

Do I need a multimeter, or is the paperclip test enough?

The paperclip test tells you if the power supply can turn on at all. The multimeter tells you if it's outputting the right voltage. The paperclip test is faster and catches most dead units. Use the multimeter if the paperclip test passes but your PC still won't boot, or if you suspect the power supply is weakening.

Can a power supply fail partially — like powering some components but not others?

Yes. A failing power supply might deliver correct voltage to some rails but not others. This is why checking all three voltage rails (12V, 5V, and 3.3V) matters. If one rail is low and the others are normal, the power supply is degrading unevenly and should be replaced.

Is it safe to touch the inside of the power supply while testing?

No. Even when unplugged, a power supply contains capacitors that can hold a dangerous charge. Never open the power supply case or touch components inside it. All testing should be done on the external connectors only.