Start with the simplest test: listen and feel

Before you plug in a multimeter or open your case, do what your senses can tell you. A working power supply makes a steady hum—not a high-pitched whine, not a grinding noise, and not silence when the PC is running. Put your hand near the fan exhaust on the back of the power supply (not blocking it). You should feel warm air moving steadily. If the fan is not spinning at all, or if it spins in bursts and stops, the power supply is struggling.

Next, check whether your PC actually powers on and stays on. Press the power button. If the system boots and runs without shutting down unexpectedly, your power supply is at least functional enough for basic operation. If it powers on for a few seconds then cuts out, or if it will not power on at all, you have a real problem—though this can also point to a bad motherboard or CPU, so do not assume the power supply is at fault yet.

Listen for clicking or popping sounds from inside the case. These are warning signs of a failing component, possibly the power supply. A single click when you first power on is normal; repeated clicking or popping is not.

Key Takeaways

  • A working power supply fan spins steadily and produces warm exhaust; a silent or stuttering fan suggests failure.
  • The paperclip test (jumping the green and black wires on the 24-pin connector) tells you whether the power supply can turn on by itself, independent of your motherboard.
  • A multimeter set to DC voltage can measure the actual output on each rail—12V, 5V, and 3.3V—to see whether they match their rated values.
  • Unexpected shutdowns, random restarts, or a PC that will not power on can come from a failing power supply, but also from a bad motherboard or CPU, so test the power supply first before replacing other parts.
  • If the power supply fails the paperclip test or produces voltages more than 5% below spec, replacement is the safest option.

The paperclip test: does the power supply turn on by itself?

This test tells you whether the power supply can actually power up without being connected to a motherboard. Unplug the power supply from the wall and from your PC entirely. Locate the 24-pin ATX connector—the large rectangular plug that connects to the motherboard. Look at the wires coming out of it. You need to find the green wire and any black wire next to it.

Take a straightened paperclip and insert one end into the socket where the green wire connects, and the other end into the socket where the black wire connects. Plug the power supply back into the wall outlet. The fan should spin up when ready. If it does, the power supply can turn on. If nothing happens, the power supply is dead and needs replacement.

Let the fan run for a few seconds, then unplug the power supply from the wall to stop it. Remove the paperclip. This test is safe—the voltages inside a power supply are dangerous, but the paperclip only bridges the control circuit, not the high-voltage section. If you are uncomfortable doing this, skip to the multimeter test instead.

Using a multimeter to measure voltage output

A multimeter is a handheld meter that measures voltage, current, and resistance. You can buy one for $15 to $30 at any hardware store or online. Set the dial to DC voltage (usually marked with a V with a line and dots, not a wavy line). The wavy line is AC voltage, which is not what you need here.

With the power supply unplugged, connect it to your PC as normal—24-pin to motherboard, 8-pin to CPU, 6-pin or 8-pin to GPU if you have one, and SATA power to any drives. Plug the power supply into the wall but do not turn on the PC yet. Touch the black probe of the multimeter to any black wire (ground), and touch the red probe to the wire you want to test. A yellow wire should read 12V. A red wire should read 5V. An orange wire should read 3.3V. If any of these read more than 5% below spec—so 12V reads below 11.4V, or 5V reads below 4.75V—the power supply is failing.

Test multiple connectors if you can. The 24-pin has all three voltages. The 8-pin CPU connector has 12V. The 6-pin GPU connector has 12V. If one connector reads low but others read correct, the problem may be a loose connection rather than a dead power supply—try reseating the connector and test again.

What to do if the PC shuts down randomly or will not start

Random shutdowns and unexpected restarts are classic signs of a failing power supply, but they can also come from an overheating CPU, a bad motherboard, or corrupted Windows. Test the power supply first because it is the easiest to rule out. Use the paperclip test or the multimeter test above. If both pass, the power supply is probably not your problem.

If the PC will not power on at all, make sure the power cable from the wall to the power supply is plugged in firmly. Check that the power supply switch on the back is set to ON (not OFF). If both are correct, try the paperclip test. If the power supply fan does not spin, the power supply is dead. If the fan does spin but the PC still will not power on, the motherboard or CPU may be at fault instead.

Keep a record of when the shutdowns happen. Do they occur during heavy gaming or video editing, or do they happen at random? Shutdowns under load point more toward a power supply that cannot deliver enough current. Shutdowns at idle point more toward a motherboard or software issue.

When to replace the power supply instead of testing further

If the paperclip test fails—the fan does not spin—replacement is your only option. A power supply that cannot turn on by itself cannot power your PC. Do not try to repair it yourself; power supplies contain capacitors that can hold a dangerous charge even after unplugging.

If the multimeter shows voltages more than 5% below spec on multiple rails, the power supply is degrading and should be replaced. A power supply that is out of spec may work for a while, but it will eventually fail, and it can damage your motherboard or other components on the way down.

If the power supply passes both tests but your PC still shuts down randomly, the power supply is probably not the cause. At that point, test your CPU temperature with a free tool like HWInfo or Core Temp. If the CPU is running above 90°C under load, clean the heatsink and reapply thermal paste. If temperatures are normal, the issue is likely the motherboard or Windows itself.

Choosing a replacement power supply

Power supplies are rated by wattage—550W, 650W, 750W, and so on. Your new power supply should be at least as powerful as your current one, and ideally 20% more powerful to give yourself headroom. A 650W supply is safe for most gaming PCs; a 750W or 850W is better if you have a high-end GPU.

Buy from a reputable brand: Corsair, EVGA, Seasonic, and Thermaltake all make reliable supplies. Avoid the cheapest no-name supplies on Amazon or eBay; they often fail quickly and can damage your other components. A good power supply costs $60 to $150 depending on wattage and efficiency rating.

Installation is straightforward: unplug the old supply, disconnect all the cables, unscrew it from the case, slide the new one in, screw it down, and reconnect the cables in the same order. If you are not comfortable opening your case, a local computer repair shop can do it for $30 to $50.

Frequently Asked Questions

Can a bad power supply damage my motherboard or GPU?

Yes. A power supply that outputs unstable or incorrect voltages can slowly degrade other components or cause them to fail suddenly. If you suspect a bad power supply, test it and replace it before it damages something more expensive. A $100 power supply is cheaper than a $300 motherboard.

Is it safe to do the paperclip test?

The paperclip test is safe because it only bridges the low-voltage control circuit. The high-voltage section of the power supply is dangerous, but the paperclip never touches it. If you are uncomfortable with the test, use a multimeter instead or take your PC to a repair shop.

What does it mean if the power supply fan spins but my PC will not turn on?

The power supply is working, but something else is preventing the PC from booting. Check that the 24-pin and 8-pin connectors are fully seated. If they are, the motherboard, CPU, or RAM may be at fault. Try reseating the RAM by removing and reinserting it firmly into its slot.

How long does a power supply usually last?

A quality power supply typically lasts 5 to 10 years under normal use. Cheaper supplies may fail in 2 to 3 years. Heat and dust shorten the lifespan, so keep your case clean and make sure the power supply fan has clear airflow.

Should I buy a modular or non-modular power supply?

Modular supplies have detachable cables, which makes cable management cleaner and improves airflow inside the case. Non-modular supplies have all cables permanently attached. Modular supplies cost a bit more but are worth it if you care about case appearance or airflow. For pure function, both work equally well.