What to check when your power supply might be dying
A failing power supply usually shows itself through sudden shutdowns, random restarts, or a computer that won't turn on at all — even though you know the wall outlet works. Before you replace it, you can run a few quick tests to confirm the power supply is the problem and not something else. Most of these checks take less than ten minutes and require only a multimeter, which costs $10 to $20 at any hardware store.
The power supply is the metal box inside your computer case that converts wall power into the different voltages your motherboard, graphics card, and drives need. When it starts to fail, it either stops delivering power altogether or delivers unstable power that causes crashes. Testing it means checking whether it's actually producing the right voltage on its cables.
Key Takeaways
- A multimeter set to DC voltage is the tool you need to test whether your power supply is producing the right electrical output.
- The 24-pin motherboard connector and the 8-pin CPU connector are the two most important cables to test first.
- A healthy power supply should show readings between 11.5 and 12.5 volts on the 12-volt rail and between 4.75 and 5.25 volts on the 5-volt rail.
- If your computer won't turn on at all, test the power supply while it's running by leaving the case open and watching for fan spin and cable voltage.
- A paperclip trick lets you test the power supply outside the computer to rule out motherboard problems.
Gather a multimeter and prepare your workspace
You will need a digital multimeter — the kind with a dial or digital display that measures voltage. Set it to DC voltage (marked as V with a straight line, not a wavy one). If your multimeter has a range selector, start at 20 volts. This setting lets you safely measure the power supply's output without damaging the meter.
Power down your computer completely and unplug it from the wall. Wait 30 seconds for the power supply capacitors to discharge. Open your case by removing the side panel — usually held by two thumbscrews or clips. You do not need to remove the power supply from the case to test it; you just need to see the cables and connectors.
Test the 24-pin motherboard connector first
The 24-pin connector is the large rectangular plug that connects the power supply to your motherboard. It has two rows of 12 pins each. Locate the black ground wires and the colored power wires — you are measuring the voltage between them.
Plug the power supply back in and turn it on. The power supply fan should spin. Touch the black multimeter probe to any black wire in the 24-pin connector. Touch the red probe to the yellow wire (12-volt rail). A healthy reading is between 11.5 and 12.5 volts. Now test the red wire (5-volt rail) with the black probe still on black — you should see between 4.75 and 5.25 volts. If either reading is outside these ranges or shows zero, the power supply is failing.
Repeat this test on the 8-pin CPU connector near the top of the motherboard. It uses the same color code: yellow for 12 volts, red for 5 volts, black for ground. The readings should match what you got from the 24-pin connector.
Check the 6-pin and 8-pin PCIe connectors if you have a graphics card
Graphics cards draw power from 6-pin or 8-pin connectors (sometimes both). These connectors are usually near the top of the card or along its edge. Test them the same way: black probe on black wire, red probe on yellow (12 volts) or red (5 volts).
If your graphics card is causing crashes or won't power on, a weak PCIe connector is often the culprit. The readings should match the motherboard connectors. If the 12-volt rail reads 11 volts or lower on the PCIe connector but 12 volts on the motherboard, the power supply is struggling under load.
Use the paperclip test if your computer won't turn on
If your computer won't power on at all, you can test the power supply in isolation using a paperclip. This tells you whether the power supply itself is dead or whether the problem is the motherboard or power button.
Unplug the power supply from the wall and from the motherboard. Straighten a paperclip into a U shape. Locate the 24-pin connector on the power supply cable (not plugged into anything). Find the green wire and any black wire in that connector. Insert one end of the paperclip into the hole behind the green wire and the other end into the hole behind a black wire. Plug the power supply back into the wall and turn on the switch on the back of the power supply. The fan should spin immediately. If it does, the power supply is working and your motherboard or power button is the problem. If the fan does not spin, the power supply is dead.
Interpret what your readings mean
Readings that are consistently low across multiple connectors — like 11 volts when you should see 12, or 4.5 volts when you should see 5 — mean the power supply is aging and losing capacity. It may still power your computer in light use but fail under load (gaming, video editing, or rendering). Readings that fluctuate wildly or drop to zero on some connectors point to a failing capacitor inside the power supply.
If all your readings are within range but your computer still crashes or won't start, the problem is likely the motherboard, a loose cable connection, or a faulty power button. Reseat all power connectors by unplugging and replugging them firmly. Make sure the 24-pin and 8-pin connectors are fully seated — they should click into place.
Know when to replace instead of test further
If the power supply fan does not spin when you turn it on, or if the paperclip test shows no fan activity, the power supply is dead and needs replacement. Do not spend time testing further. A power supply that produces zero voltage on all rails is not repairable.
If your readings are low but not zero, and your computer is more than five years old, replacement is usually cheaper and safer than troubleshooting. Power supplies degrade over time, and a weak one can damage your motherboard or storage drives if it fails suddenly. A new power supply costs $50 to $150 depending on wattage and quality.
Frequently Asked Questions
Do I need to remove the power supply from the case to test it?
No. You can test it while it is still mounted in the case as long as you can reach the connectors and see the fan. Testing it in place is actually safer because you avoid the risk of damaging components while moving it.
What if my multimeter shows 12 volts on one test and 11.8 on another?
Small variations like that are normal. Power supplies are designed to tolerate readings between 11.5 and 12.5 volts on the 12-volt rail. If you see 11.8 volts consistently, the power supply is fine. If the reading drops to 10 volts or lower, it is failing.
Can a bad power supply damage my motherboard?
Yes. A power supply that delivers unstable or excessive voltage can fry capacitors on your motherboard or corrupt data on your drives. If you suspect a power supply failure, stop using the computer until you have tested it or replaced it.
My power supply fan spins but my computer won't turn on. Is the power supply bad?
Not necessarily. A spinning fan means the power supply has power, but it does not mean it is delivering the right voltage to the motherboard. Use your multimeter to check the 24-pin and 8-pin connectors. If the voltages are in range, the problem is your motherboard or power button, not the power supply.
How often should I test my power supply?
You only need to test it if your computer is crashing, restarting randomly, or won't turn on. A healthy power supply does not need routine testing. If your computer is running normally, your power supply is fine.