What a PSU does and why checking it matters

Your power supply unit (PSU) is the box inside your computer that converts electricity from the wall outlet into the right voltage and current for each component. When a PSU fails, your computer won't turn on at all, or it will shut down randomly during use. Before you assume your motherboard or hard drive is broken, a few quick checks can tell you whether the PSU itself is the problem.

Most PSU failures happen gradually — you might notice your computer restarting without warning, or fans spinning up and down erratically. Other times a PSU dies completely and your machine won't power on no matter what you do. The checks below work whether your PSU is dead or just struggling.

Key Takeaways

  • The simplest first check is the power button on the back of the PSU itself — flip it off and on to rule out an accidental shutdown.
  • Listen and feel for the fan inside the PSU when the computer is running; a PSU with a dead fan often overheats and shuts down.
  • A multimeter can test whether the PSU is actually sending power to the motherboard, but you need to know which pins to probe.
  • If your PSU is still under warranty, contact the manufacturer before opening it — opening voids most warranties and PSUs contain dangerous voltage even when unplugged.
  • Swapping in a known-working PSU from another computer is the fastest way to confirm whether yours is the problem.

The physical checks you can do without tools

Start with the power button on the back of the PSU itself — it is a small black or red switch. Make sure it is in the ON position. This sounds obvious, but it catches people regularly, especially after a power outage or if someone bumped the switch while cleaning.

Next, look at the cables running from the PSU to your motherboard and graphics card. The main connector to the motherboard is usually a large rectangular plug with 20 or 24 pins. The graphics card gets a separate cable, often with 6 or 8 pins. Push each connector in firmly — a loose connection will prevent your computer from starting or cause random shutdowns. You should hear a click when the connector seats properly.

While the computer is running (or attempting to run), put your hand near the PSU fan opening and feel for air movement. A working PSU will push warm air out the back. If you feel nothing, or if the fan is not spinning at all, the PSU is likely overheating and shutting itself down as a safety measure. Listen for a humming or whirring sound from inside the PSU — silence usually means the fan is stuck or the PSU is dead.

Testing with a multimeter

A multimeter is a handheld device that measures voltage, current, and resistance. You can buy one for $10 to $30 at any hardware store or online. To test your PSU, you need to turn off the computer, flip the PSU power switch to OFF, and then unplug the PSU from the wall. Wait 30 seconds for the capacitors inside to discharge — this is important for your safety.

Set your multimeter to measure DC voltage (usually marked with a V with a straight line above it). Locate the 24-pin motherboard connector. You are looking for the pins that carry power — typically the red wires carry +12V, the black wires are ground, and the yellow wires carry +5V. Touch the multimeter's red probe to a red wire pin and the black probe to a black wire pin. A healthy PSU will show approximately 12 volts. Repeat this test with a yellow wire and a black wire — you should see approximately 5 volts.

If the multimeter shows zero volts, or if the reading jumps around, the PSU is not delivering stable power. If the readings are correct but your computer still won't start, the problem is likely elsewhere in your system.

The swap test — the fastest confirmation

If you have access to another computer that is currently working, borrow its PSU and install it in your machine. This is the most reliable way to know whether your PSU is the culprit. Power off your computer completely, unplug it, and wait 30 seconds. Unscrew the side panel of your case and locate the PSU — it is usually in the bottom rear corner.

Disconnect all the cables from your current PSU: the motherboard connector, any graphics card connectors, and the SATA power cables to your hard drives or SSDs. Unscrew the PSU from the case (usually four screws) and slide it out. Install the borrowed PSU in the same position, reconnect all the cables in the same places, and screw it back in. Power on the computer.

If your computer starts and runs normally with the borrowed PSU, your original PSU is the problem. If the computer still will not start, the issue is something else — the motherboard, CPU, RAM, or another component. Return the borrowed PSU and investigate further or take your machine to a technician.

When the PSU fan is stuck or making noise

A PSU fan that is stuck or grinding usually means the bearing is worn out or debris is blocking the blade. Do not try to oil the bearing or force the fan to spin — this will damage it further. A stuck fan means the PSU will overheat and shut down, sometimes within minutes of powering on.

Some PSUs have a warranty that covers fan replacement. Contact the manufacturer with your PSU model number (printed on the label) and ask whether they offer a replacement or repair. Many will send you a new unit for a flat fee or for free if it is still under warranty. If the warranty has expired, replacing the PSU is usually cheaper and safer than trying to repair it yourself.

What to do if the PSU is definitely broken

Once you have confirmed the PSU is the problem, you need a replacement. Check the wattage of your current PSU — it is printed on the label on the side or back. Most desktop computers use 500W to 750W. If you have a graphics card, look up its power requirements and add that to your CPU's power draw to make sure the new PSU is large enough.

Buy a PSU from a reputable manufacturer — Corsair, EVGA, Seasonic, and Thermaltake are common choices. Avoid the cheapest option; a low-quality PSU can damage your other components. Installation is the reverse of removal: unplug the old PSU, disconnect all cables, unscrew it, slide in the new one, reconnect the cables, and screw it back in.

Frequently Asked Questions

Can I test a PSU while it is still plugged in?

No. A PSU contains capacitors that hold a dangerous electrical charge even after you unplug it. Always unplug the PSU from the wall and wait 30 seconds before touching any internal components or testing with a multimeter. If you are not comfortable working inside your computer, take it to a technician instead.

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

The fan spinning means the PSU has power, but it does not mean the PSU is delivering the correct voltage to your motherboard. Use a multimeter to test the voltage on the motherboard connector, or try the swap test with a known-working PSU. The problem could also be a bad motherboard, RAM, or CPU.

Is it safe to open a PSU to clean the fan?

No. Opening a PSU voids the warranty and exposes you to dangerous voltage. Even unplugged, a PSU can hold a charge. If the fan is stuck, contact the manufacturer about a replacement or buy a new PSU. Cleaning the outside vents with a soft brush is safe, but do not open the case.

How long does a PSU usually last?

Most PSUs last five to ten years with normal use. Dust buildup, high temperatures, and power surges shorten the lifespan. If your PSU is more than seven years old and showing signs of failure, replacement is usually the best option rather than repair.

Can a bad PSU damage other parts of my computer?

Yes. A failing PSU can send unstable voltage to your motherboard, graphics card, or storage drives, which can corrupt data or damage the components permanently. If you suspect a PSU problem, stop using the computer and test it or have it tested before running it for extended periods.