Signs Your Power Supply Is Dying
A failing power supply usually announces itself before it stops working entirely. The most common warning signs are a computer that shuts down without warning, restarts on its own during normal use, or fails to turn on at all even though the outlet works. You might also hear a loud humming, buzzing, or grinding noise from inside the case—that's often the cooling fan wearing out or internal components breaking down.
Another telltale sign is a burning smell near the back of the computer where the power supply sits. If you notice this, unplug the machine when ready and do not use it until the power supply has been replaced. A smell like burnt plastic or charred electronics means the power supply has already begun to fail and poses a fire risk.
Some failures are less dramatic. Your computer might freeze randomly, run slower than usual, or peripherals like USB devices might stop working without explanation. These can point to a power supply that is no longer delivering stable voltage to all the components inside.
Key Takeaways
- Unexpected shutdowns, random restarts, and failure to power on are the most common signs of a failing power supply.
- Loud buzzing, grinding, or humming sounds from the power supply fan indicate internal wear or damage.
- A burning smell means the power supply is actively failing and should be unplugged when ready.
- Unstable voltage can cause freezing, slowdowns, or USB devices to stop responding, even if the computer still turns on.
- Testing with a multimeter or swapping in a known-good power supply are the most reliable ways to confirm failure.
How to Test a Power Supply With a Multimeter
A multimeter is an inexpensive tool (usually $15 to $30) that measures electrical voltage. You can use one to check whether your power supply is delivering the correct voltage to your motherboard. Set the multimeter to DC voltage mode, then touch the red probe to the positive wire and the black probe to the ground (usually a black wire) on any connector inside the case while the computer is powered on.
The main 24-pin connector that plugs into the motherboard should read between 11.4 and 12.6 volts on the yellow wires (12V rail) and between 4.75 and 5.25 volts on the red wires (5V rail). If the readings are significantly lower or the voltage fluctuates wildly, the power supply is failing. If you are not comfortable opening the case or working with electricity, skip this step and move to the swap test instead.
Do not touch any components inside the case while the computer is running, and always unplug the machine before opening it. If you are uncertain about any step, ask someone with computer repair experience to do the testing for you.
The Swap Test: The Simplest Confirmation
The most straightforward way to know if your power supply is bad is to borrow a working power supply of the same wattage and connector type from a friend or colleague and install it in your computer. If the borrowed power supply solves the problem—no more shutdowns, no more noise, no more freezing—then your original power supply is the culprit.
This test works because it removes all guesswork. If the computer runs normally with the replacement power supply, you have your answer. If the same problems persist, the issue lies elsewhere in your system, and you should investigate other components like the motherboard or RAM.
The swap test does require you to open the case and unplug connectors, which takes about 10 minutes if you have done it before. If you have never opened a computer case, watch a video for your specific model first, or ask a repair shop to do the swap for you.
When to Replace Rather Than Repair
Power supplies are not designed to be repaired by users. Unlike a hard drive or RAM stick, a power supply contains capacitors and transformers that hold electrical charge even after you unplug the machine. Opening one can result in serious injury, and the internal components are not sold separately in a way that makes repair practical.
Replacement is the only safe option. A new power supply for a desktop computer typically costs between $50 and $150 depending on wattage and quality. Laptop power supplies are usually $30 to $80. If your computer is very old, you may want to weigh the cost of a new power supply against the age and condition of the rest of the machine.
If you are not comfortable installing a new power supply yourself, a local computer repair shop can do it for $50 to $100 in labor. The total cost is usually still less than replacing the entire computer.
How to Choose the Right Replacement Power Supply
When you buy a replacement, you need to match two things: the wattage and the form factor (the physical size and connector layout). Your current power supply has both printed on a label on the side. For a desktop, common wattages are 450W, 550W, 650W, and 750W. For a laptop, the wattage is usually printed on the power brick itself.
The form factor for desktops is almost always ATX (the standard size) or SFX (smaller, for compact cases). Laptops use proprietary connectors specific to the brand and model. When you order a replacement, search for your exact computer model plus "power supply" to find the right part.
You can safely buy a power supply with slightly higher wattage than the original—for example, a 650W supply to replace a 550W—but do not go lower. Your computer will only draw the power it needs, so oversizing is harmless. Undersizing can cause instability or failure.
Preventing Power Supply Failure
Power supplies fail faster in hot environments and when they are clogged with dust. Keep your computer in a cool room with good airflow, and clean the dust filters or vents every three to six months. A can of compressed air works well for this—spray short bursts from a distance of 6 inches, holding the can upright.
Avoid plugging your computer into a power strip with many other devices. If your home experiences frequent power surges or brownouts, use a surge protector or uninterruptible power supply (UPS) to shield the power supply from voltage spikes. A UPS also keeps your computer running for a few minutes during a power outage, giving you time to save your work and shut down properly.
Do not block the vents on the back of the computer case, and make sure there is at least 3 inches of clear space behind it for air to flow. A power supply that runs too hot will fail years sooner than one that stays cool.
When to Call a Professional
If you are uncomfortable opening your computer case, working with electrical components, or using a multimeter, take the machine to a repair shop and describe the symptoms. A technician can diagnose the power supply in minutes and install a replacement if needed. This is the safest route if you have any doubt about your ability to do it yourself.
You should also call a professional if the computer is still under warranty. Opening the case yourself may void the warranty, so check the documentation first. If the machine is under coverage, contact the manufacturer or the retailer where you bought it.
Frequently Asked Questions
Can a bad power supply damage other parts of my computer?
Yes. A failing power supply can deliver unstable voltage that damages the motherboard, RAM, or storage drives over time. If you suspect power supply failure, stop using the computer to avoid cascading damage to other components. The sooner you replace it, the safer your other hardware is.
Why does my computer shut down suddenly without any warning?
Sudden shutdown without a warning message usually means the power supply cannot deliver enough power under load. When the CPU or GPU demands more electricity, the power supply cuts out to protect itself. This is one of the most common power supply failure symptoms.
Is it safe to use a computer with a failing power supply?
No. A failing power supply can overheat, catch fire, or damage other components. If you smell burning plastic, hear loud grinding, or see sparks, unplug when ready. Even if the symptoms are mild, replace the power supply as soon as you can.
How long does a power supply usually last?
Most desktop power supplies last 5 to 10 years with normal use. Laptop power supplies often fail sooner, typically 3 to 5 years, because they run hotter and are handled more frequently. The lifespan depends on how much heat the supply is exposed to and how often it is used.
Can I use any power supply in my computer?
No. You must match the wattage and form factor to your computer. Using the wrong connector type will not fit, and using too low a wattage will cause instability. Check the label on your current power supply and search for an exact replacement or a compatible model from a reputable brand.