What a capacitor does and why it fails
A capacitor is a component that stores electrical charge temporarily, like a small battery that charges and discharges in milliseconds. In your computer, power supply, or monitor, capacitors smooth out power delivery, filter noise, and help circuits start up. When a capacitor fails — usually because it ages, overheats, or gets damaged — the device it powers either stops working or behaves erratically.
You can test whether a capacitor is working or dead using a multimeter, a handheld tool that measures voltage, resistance, and other electrical properties. The test takes about a minute and tells you whether the capacitor is holding a charge, leaking, or completely failed. This matters because a bad capacitor in a power supply can damage other components, and replacing it costs far less than replacing the whole unit.
Key Takeaways
- A multimeter set to resistance mode (ohms) can show whether a capacitor is working, leaking, or dead by measuring how it charges and discharges.
- You must discharge the capacitor completely before testing it, or you risk a shock and damaging the multimeter.
- A good capacitor shows a rising resistance reading that climbs toward infinity; a dead one stays at zero or very low resistance.
- Capacitors can look fine on the outside but be internally failed, so a visual inspection is not enough to confirm the problem.
Discharge the capacitor before you touch it
A charged capacitor can hold enough voltage to hurt you or destroy a multimeter, even if the device is unplugged. Before you test anything, you must drain all the stored charge. The safest way is to use an insulated screwdriver to short the two terminals together — touch the metal tip to both leads at the same time for a second or two. You should see a small spark; that is the charge leaving.
If you are working inside a power supply or monitor, look for a resistor (a small cylinder with colored bands) connected across the capacitor terminals. That resistor is there to bleed off charge slowly. Wait at least five minutes after unplugging the device before you touch anything, then short the terminals anyway to be sure. If you skip this step and the capacitor is still charged, the discharge will arc across your screwdriver and possibly across your hand.
Set your multimeter to resistance mode
Turn the multimeter dial to the resistance setting, marked with the Greek letter omega (Ω). If your multimeter has multiple resistance ranges — usually labeled 200Ω, 2kΩ, 20kΩ, 200kΩ, or 2MΩ — start with the highest range (2MΩ). This protects the meter if something goes wrong. You can switch to a lower range later if you need a more precise reading.
Touch the two multimeter probes to the two capacitor terminals. It does not matter which probe touches which terminal — resistance works the same in both directions. Watch the display as you hold the probes in place. A healthy capacitor will show the resistance climbing upward, starting near zero and rising toward infinity (usually displayed as "1" or a very high number). This climbing happens because the capacitor is charging through the multimeter's internal battery.
Interpret what the reading tells you
A good capacitor shows a rising resistance that climbs steadily and then levels off at a very high value — often above 1 million ohms. The climb takes a few seconds. This behavior means the capacitor is charging and holding the charge, which is what it should do.
A dead or shorted capacitor shows zero or very low resistance (under 100 ohms) that does not change. This means the capacitor cannot hold a charge and is leaking electricity constantly. It needs to be replaced.
A leaking capacitor shows a resistance that climbs but only partway — it might reach 100,000 ohms and then stop, or it might climb very slowly. This means the capacitor is degraded and losing its ability to hold charge. Leaking capacitors often cause the same problems as dead ones, and they should be replaced.
What to do if the test shows a bad capacitor
If the capacitor failed the test, you have found the problem. Take a photo of the capacitor's markings — the voltage rating and capacitance value (measured in microfarads, or µF) — before you remove it. You can order a replacement online or from an electronics supplier using those numbers. The replacement must have the same voltage rating and capacitance value, though the physical size can be slightly different.
Removing and soldering in a new capacitor requires a soldering iron and basic soldering skill. If you are not comfortable with that, a repair shop can do it for you. The cost of a replacement capacitor is usually between five and twenty dollars, plus labor if you have someone else install it. This is far cheaper than replacing the entire power supply or monitor.
When a capacitor test is not enough
A multimeter test tells you whether the capacitor is holding charge, but it does not test whether the capacitor is the actual cause of your device's problem. A device can have a bad capacitor and still work poorly for other reasons — a failing power supply might have multiple bad capacitors, or the problem might be elsewhere entirely.
If you replace a capacitor and the device still does not work, the issue is somewhere else. Bulging or leaking capacitors are easier to spot visually — if you see a capacitor with a domed or split top, or brown liquid around the base, it is definitely bad. But many failed capacitors look normal on the outside, so a multimeter test is the only way to know for sure.
Frequently Asked Questions
Can I test a capacitor while it is still soldered to the board?
Not reliably. Other components on the board will interfere with the reading and give you false results. You need to desolder the capacitor first — remove at least one lead from the circuit — so the multimeter only measures the capacitor itself.
What if my multimeter does not have a resistance setting?
Most analog and digital multimeters have a resistance mode. If yours does not, you have a very basic meter that is not suitable for testing capacitors. A basic multimeter costs fifteen to thirty dollars and will do this job and many others.
Is it safe to test a capacitor in a powered-on device?
No. Always unplug the device first and wait several minutes. Even then, discharge the capacitor manually before you touch it. Testing a live circuit can damage the multimeter and hurt you.
What does it mean if the resistance climbs and then drops back down?
That usually means the capacitor is leaking or the multimeter probes are not making good contact with the terminals. Try cleaning the terminals with a dry cloth and test again. If it still drops, the capacitor is failing.
Can a capacitor fail and then work again?
Not permanently. A capacitor that has failed will not recover on its own. If a device works intermittently and you suspect a bad capacitor, the capacitor is probably degrading — it works sometimes and fails other times depending on temperature and load. Replace it.