What you're actually checking when you test a PSU

Your power supply unit (PSU) is the box inside your computer that converts electricity from the wall into the right voltage for each component. When something goes wrong with it, your PC either won't turn on at all, shuts down randomly, or won't stay stable under load. Testing it means checking whether it's delivering power correctly and whether it's failing.

You have three practical ways to check: listen and look for obvious failure signs, use a multimeter to measure the actual voltage it's producing, or swap in a known-working PSU to see if the problem goes away. The first method catches most failures. The second requires a cheap tool and some care. The third is the most reliable but also the most time-consuming.

Key Takeaways

  • A PSU that won't turn on your PC, makes a burning smell, or has a visibly bulging capacitor has failed and needs replacement.
  • Random shutdowns or instability under heavy use often point to a PSU that's failing gradually, which a multimeter can help confirm.
  • A multimeter costs $10 to $20 and lets you measure whether the PSU is outputting the correct voltage to your motherboard and drives.
  • The most reliable test is swapping in a different PSU, because it rules out the power supply as the cause without any guesswork.
  • Before opening your case, unplug the PSU from the wall and press the power button to discharge any remaining electricity.

Visual and physical checks you can do right now

Before you touch anything, look at the back of your PSU for obvious damage. If you see a burn mark, smell something burnt when the PC is on, or notice the fan isn't spinning when the computer is running, the PSU has likely failed. Unplug it when ready and do not use it.

If your PC won't turn on at all, check that the power cable is plugged into both the wall and the PSU, and that the wall outlet itself works (plug a lamp into it to test). Then look at the back of the PSU for a small rocker switch. If it's in the off position, flip it to on. This catches more people than you'd expect.

If the PC still won't start, unplug the PSU from the wall, wait 30 seconds, then plug it back in and try again. This resets the PSU's internal protection circuits, which sometimes trip if there was a power surge.

Opening the case safely to inspect inside

If you need to look inside the case or use a multimeter, you must discharge any electricity stored in the PSU first. Unplug the power cable from the wall, then press and hold your PC's power button for 10 seconds. This drains the capacitors inside the PSU.

Once that's done, you can open the case. Look at the capacitors inside the PSU — they're the cylindrical components that look like small cans. If any of them are bulging at the top, leaking a brown or black liquid, or visibly cracked, the PSU has failed internally and must be replaced. Do not attempt to repair it.

While you're inside, also check that the 24-pin power connector to the motherboard and the 8-pin CPU power connector (usually near the top of the motherboard) are fully seated and not loose. A loose connector can cause the same symptoms as a failing PSU.

Using a multimeter to measure PSU output

A multimeter is a handheld tool that measures voltage, and it's the most practical way to confirm whether your PSU is outputting the right power. You can buy one for $10 to $20 at any hardware store or online. Set it to DC voltage (usually marked with a V with a line and dots, not a wavy line).

With the PSU unplugged and discharged as described above, locate the 24-pin motherboard connector. This is the large flat plug with many wires. You're going to measure the voltage between specific pins without disconnecting anything. Touch the multimeter's black probe to any black wire (ground), then touch the red probe to a red wire (5V) and note the reading — it should be between 4.75V and 5.25V. Then test a yellow wire (12V) — it should read between 11.4V and 12.6V.

If the readings are way off — say, 3V instead of 5V, or 8V instead of 12V — the PSU is failing. If the readings are correct, the PSU is likely fine and the problem is elsewhere. If you're not comfortable doing this, skip it and move to the swap test instead.

The swap test: the most reliable method

The fastest way to know for certain whether the PSU is the problem is to borrow a working PSU from a friend or take your PC to a shop and test it with a different unit. If the PC works fine with the other PSU, yours has failed. If the same problem happens with the replacement, the PSU is not the cause.

To swap PSUs, unplug everything, open the case, and disconnect all the power cables from the PSU to the motherboard, CPU, graphics card, and drives. There are usually two or three clips holding the PSU in the case — release them and slide the PSU out. Install the new one in reverse order, making sure all connectors are fully seated before closing the case.

This method is reliable because it removes all doubt. It does take more time and requires access to another PSU, but if you're stuck between "is it the power supply or something else?" this answers the question definitively.

When to replace versus when to keep testing

If your PC won't turn on at all and the visual checks found nothing, or if it randomly shuts down under load, the PSU is a likely culprit but not certain. Random shutdowns can also come from an overheating CPU, a failing hard drive, or bad RAM. The multimeter test or swap test will narrow it down.

If the multimeter shows low voltage, or if swapping the PSU fixes the problem, you need a replacement. PSUs cannot be repaired in the field — they either work or they don't. When buying a replacement, match or exceed the wattage of the original (the wattage is printed on the label) and make sure the connectors match your motherboard and components.

Frequently Asked Questions

Can a PSU fail partially and still turn the PC on?

Yes. A failing PSU might power on the motherboard but not deliver enough power to a graphics card, causing crashes during games. Or it might work fine at idle but fail under heavy load. This is why the multimeter test and swap test are useful — they catch partial failures that visual inspection misses.

Is it safe to test a PSU with a multimeter?

It's safe as long as the PSU is unplugged from the wall and you've discharged it by holding the power button. You're only measuring voltage on the output connectors, not touching anything inside the PSU itself. If you're uncomfortable with it, the swap test is safer and just as reliable.

What if the PSU fan doesn't spin?

Many PSUs have a feature where the fan doesn't spin until the unit gets hot enough to need cooling. If the PC is running fine and the PSU isn't hot, this is normal. If the PSU is hot and the fan still won't spin, or if the PC is shutting down, the PSU has likely failed.

How long does a PSU usually last?

Most PSUs last 5 to 10 years depending on how much heat they're exposed to and how hard they work. If your PSU is more than 8 years old and showing signs of failure, replacement is usually the right choice rather than spending time troubleshooting.

Do I need to replace the whole PSU or just repair it?

You replace the whole unit. PSUs contain high-voltage components that can store electricity even when unplugged, and repairing them requires specialized equipment and training. Replacement is safer and usually cheaper than repair.