How to test your PSU before you assume it's broken

A failing power supply often looks like a computer that won't turn on, random shutdowns, or components that stop working without warning. Before you replace it, you can run a few quick tests to confirm the PSU is actually the problem. The most reliable method is the paperclip test, which powers the PSU without a computer attached. You can also check for physical damage, listen for fan noise, and use your computer's built-in tools to monitor power delivery while it runs.

Most of these tests take under five minutes and require no special equipment beyond a paperclip and a multimeter if you want to measure voltage. If your PSU passes these checks, the problem lies elsewhere — usually in the motherboard, CPU, or a loose cable connection.

Key Takeaways

  • The paperclip test powers your PSU outside the computer to see if the fan spins and the unit produces power without any load.
  • A multimeter can measure the actual voltage coming from each cable, which tells you whether the PSU is delivering the correct power levels.
  • Windows and Linux both include built-in tools to monitor CPU temperature and power draw, which can reveal whether the PSU is struggling under load.
  • Physical signs like burn marks, a burning smell, or a completely silent fan usually mean the PSU needs replacement rather than testing.
  • A PSU that works fine at idle but shuts down under heavy load is often failing and should be replaced, even if the paperclip test passes.

The paperclip test: powering the PSU alone

The paperclip test is the fastest way to check whether your PSU can produce power at all. You bridge two pins on the 24-pin motherboard connector with a paperclip, which tells the PSU to turn on without needing the computer's power button. If the fan spins and the unit stays on, the PSU has basic function. If nothing happens, the PSU is likely dead.

To run the test, unplug the PSU from the wall and from all components. Locate the 24-pin connector (the large rectangular plug that connects to the motherboard). Looking at the connector from the front, find the green wire and any black wire next to it — usually the green wire is in the middle-left area. Insert a paperclip so it touches both the green wire's pin and an adjacent black wire's pin. Plug the PSU back into the wall. The fan should spin within a few seconds. If it does, unplug immediately and remove the paperclip.

A spinning fan means the PSU can turn on and produce at least some power. A completely silent PSU, or one that makes a clicking sound and then stops, suggests an internal failure. Note that some modern PSUs have very quiet fans that may not spin at all under light load — if you hear any sound or see any LED light up, the unit is responding.

Checking cables and connectors for damage

Before running any electrical test, look for visible damage that would make testing unsafe. Inspect the power cable for cuts, burns, or exposed wire. Check the PSU itself for scorch marks, melted plastic, or discoloration around the vents. Smell near the PSU — a burning or acrid odor means stop immediately and do not power it on again.

Look at every connector: the 24-pin motherboard plug, the 8-pin CPU power connector, and any 6-pin or 8-pin PCIe connectors for graphics cards. Bent pins, corrosion, or loose wires inside the connector are signs of damage. Gently wiggle each connector where it plugs into the PSU — if it moves too freely or feels loose, the connection may be failing. A loose connection can cause intermittent power loss that looks like a PSU failure but is actually just a bad plug.

If you find physical damage, do not test the PSU further. Damaged power supplies can cause fires or electrocution. Replace it instead.

Using a multimeter to measure voltage

A multimeter measures the actual voltage coming from each PSU cable. This tells you whether the PSU is delivering the correct power levels — typically 12 volts, 5 volts, and 3.3 volts depending on the cable. You can buy a basic digital multimeter for $15 to $30 at any hardware store.

Set the multimeter to DC voltage mode (usually marked with a V with a line underneath). Unplug the PSU from the wall. Locate the 24-pin connector and identify the wire colors: red wires carry 5 volts, yellow wires carry 12 volts, orange wires carry 3.3 volts, and black wires are ground. Plug the PSU back in without the paperclip test — just plug it into the wall. Touch the multimeter's black probe to any black wire and the red probe to a red wire. The reading should be close to 5 volts. Repeat with a yellow wire (should read close to 12 volts) and an orange wire (should read close to 3.3 volts).

Readings within 10 percent of the target voltage are normal — so 4.5 to 5.5 volts for the 5-volt rail, 10.8 to 13.2 volts for the 12-volt rail. If readings are far outside this range or show zero, the PSU is not delivering power correctly. Unplug immediately and do not use the PSU.

Monitoring power and temperature while the computer runs

If your PSU passes the paperclip test and voltage test, but your computer still shuts down or behaves strangely, the problem may only appear under load. Windows and Linux both include tools to monitor CPU temperature and power draw in real time.

On Windows, open Task Manager (press Ctrl+Shift+Esc), go to the Performance tab, and click CPU. You will see a temperature reading in the bottom right. Most CPUs run between 30 and 60 degrees Celsius during normal use and up to 90 degrees under heavy load. If temperature climbs above 95 degrees or the computer shuts down when temperature reaches a certain point, the PSU may not be delivering enough power to keep the system stable, or the CPU cooler is failing. On Linux, use the command watch sensors in a terminal to see real-time temperature data.

To stress-test the PSU, run a demanding program like a video game, video encoding software, or a benchmark tool. If the computer shuts down or restarts during heavy use but works fine for everyday tasks, the PSU is likely failing under load. A PSU that works at idle but fails under stress should be replaced, even if it passes the paperclip test.

Signs that point to a different problem

Not every computer that won't turn on has a bad PSU. Before you replace it, rule out other common causes. Check that the power cable is firmly plugged into both the wall outlet and the PSU. Try a different outlet — some outlets are controlled by a switch or are on a circuit that has tripped. Plug a lamp or phone charger into the outlet to confirm it has power.

If the PSU fan spins but the computer does not start, the problem is usually the motherboard, not the PSU. If the computer starts but shuts down after a few minutes, it could be overheating rather than power delivery — check that the CPU cooler is seated properly and that dust is not blocking the vents. If only one component stops working (like a graphics card or hard drive), that component may be failing rather than the PSU.

A computer that powers on and off repeatedly, or that powers on but displays nothing on screen, often points to a motherboard or RAM problem. These symptoms can look like a PSU issue but usually are not.

When to replace the PSU instead of testing it

Some signs mean you should replace the PSU without testing. If you see burn marks, smell burning plastic, or hear a loud pop or crack, stop using the computer immediately. Do not attempt to test a PSU that shows physical damage — it can cause electrical shock or fire.

If the PSU is more than seven or eight years old and has started failing, replacement is usually cheaper and safer than repair. Older PSUs lose efficiency over time and are more likely to fail suddenly. If you have already ruled out other causes and the PSU fails the paperclip test or voltage test, replacement is the right choice.

When you buy a replacement, match the wattage to your system's needs. A graphics card and CPU together typically draw 300 to 600 watts under full load. A PSU rated for 650 to 750 watts is safe for most gaming or workstation builds. Look for 80+ Bronze certification or higher, which means the PSU meets efficiency standards and is less likely to fail prematurely.

Frequently Asked Questions

Can I test a PSU while it's still plugged into the computer?

No. Always unplug the PSU from all components before running the paperclip test or using a multimeter. Testing while connected can damage the motherboard or other parts. The only exception is monitoring temperature and power draw through software, which requires the computer to be running normally.

What does it mean if the PSU fan spins but the computer won't start?

The PSU is producing some power, but the computer is not responding. This usually points to a motherboard, CPU, or RAM problem rather than the PSU. Check that all cables are fully seated, try removing and reseating the RAM, and look for any LED lights or beep codes from the motherboard that might indicate what is wrong.

Is it safe to touch the PSU while testing it?

The PSU contains high-voltage capacitors that can hold a charge even when unplugged. Do not open the PSU case or touch internal components. You can safely touch the outside of the case and the connectors. If you need to measure voltage with a multimeter, touch only the metal pins on the connectors, not the wires themselves.

How long should I run the paperclip test?

Run it for just 10 to 15 seconds. The paperclip test is only meant to confirm the PSU can turn on — it is not designed to run the PSU for extended periods without a load. Unplug the PSU and remove the paperclip as soon as you confirm the fan is spinning.

Can a PSU fail partially, where some cables work but others don't?

Yes. A PSU can deliver correct voltage on the 12-volt rail but fail on the 5-volt rail, or vice versa. This is why measuring voltage on multiple cables with a multimeter is useful — it shows which rails are failing. A PSU with partial failure should be replaced, as it will cause instability or damage to components that depend on the failed rail.