What a capacitor does and why it fails

A capacitor is a component that stores electrical charge temporarily. It sits in power supplies, audio equipment, motors, and dozens of other devices. When a capacitor fails, the device often stops working or behaves erratically — a power supply might not turn on, a speaker might sound distorted, or a motor might not start.

Capacitors fail for several reasons: age, heat damage, electrical overload, or manufacturing defects. The longer a device runs, the more likely its capacitors will degrade. Testing a capacitor tells you whether it is still functional or needs replacement.

You can test a capacitor in two ways: with the power on (live testing, which is riskier) or with the power off (safer and more common). Most people use a multimeter — a handheld meter that measures voltage, resistance, and capacitance — because it is inexpensive and works for most situations.

Key Takeaways

  • A multimeter set to capacitance mode can measure whether a capacitor holds charge, but only when the device is powered off and the capacitor is discharged.
  • Before testing, discharge the capacitor by touching a screwdriver across its leads to prevent electric shock and damage to the meter.
  • A reading close to the capacitor's rated value (printed on its body) means it is likely working; a reading of zero or far below the rated value means it has failed.
  • Some capacitors are too large or too specialized for a multimeter to test accurately, and those require a dedicated capacitor tester or visual inspection for obvious damage like bulging or leaking.

Discharging the capacitor safely

Before you touch a capacitor with a meter or your hands, you must discharge it. A charged capacitor can deliver a painful or dangerous shock, even if the device is unplugged. Discharging takes seconds and is the most important safety step.

Locate the capacitor on the circuit board or inside the device. It usually looks like a small cylinder with two metal leads (wires) sticking out, or two flat terminals on top. Turn off and unplug the device completely. Wait at least one minute for any residual charge to bleed away.

Take an insulated screwdriver (one with a plastic handle) and touch the metal tip across both leads of the capacitor at the same time. You may see a small spark or hear a tiny pop — that is the capacitor discharging, and it is normal. Do this once or twice to be sure. The capacitor is now safe to handle.

Testing with a digital multimeter

A digital multimeter is the standard tool for this job. You can buy one for $15 to $50 at any hardware store or online. Set the dial to the capacitance setting, usually marked with a symbol that looks like two parallel lines (||) or the letters "F" for farads.

Touch the red probe to the positive lead of the capacitor and the black probe to the negative lead. (If the capacitor is unmarked, it does not matter which probe goes where.) The meter will display a number. Wait a few seconds — some meters take time to stabilize.

Compare the reading to the value printed on the capacitor's body. Capacitor values are measured in microfarads (µF), nanofarads (nF), or picofarads (pF). A working capacitor will read within about 20 percent of its rated value. For example, a 10 µF capacitor should read between 8 and 12 µF. A reading of zero, or a value far below the rated amount, means the capacitor has failed.

Reading the capacitor label

Every capacitor has a label or printing on its body that tells you its rated capacitance. This is the number you compare your meter reading against. The label might say "10µF", "100nF", "2.2uF", or similar — the unit (µF, nF, or pF) matters.

Some capacitors also show a voltage rating, like "25V" or "450V". This tells you the maximum voltage the capacitor can safely handle. It does not affect your test, but it is useful information if you need to replace the capacitor — you must use one with the same or higher voltage rating.

If the label is worn or missing, you can sometimes find the capacitor's value by searching the device's schematic online or by looking at nearby components and circuit traces. If you cannot find the value, you can still test the capacitor by looking for visible damage (see below) or by using a dedicated capacitor tester.

What the meter readings mean

A reading very close to the rated value — within 20 percent — means the capacitor is working normally. You can put the device back together and test it.

A reading significantly lower than the rated value, but not zero, suggests the capacitor is degraded. It may still work in some situations, but it is weakening and could fail soon. In critical circuits (like power supplies), replace it. In less critical circuits (like audio filters), you might get away with leaving it in place temporarily.

A reading of zero or "OL" (overload) means the capacitor is open — it has failed completely and will not hold charge. Replace it. An extremely high reading (much higher than the rated value) is rare and usually means the meter is not making good contact with the leads. Clean the leads with a dry cloth and try again.

When a multimeter is not enough

Some capacitors are too large or too specialized for a standard multimeter to test. Electrolytic capacitors larger than 100 µF, film capacitors, and high-voltage capacitors sometimes give inaccurate readings or no reading at all on a multimeter.

In these cases, look for visible damage instead: a bulging or domed top, a split seam, or a brown or white crusty substance leaking from the bottom. Any of these signs means the capacitor has failed and needs replacement. If the capacitor looks fine but you still suspect it is bad, you can buy a dedicated capacitor tester (sometimes called an ESR meter) for $30 to $100, or take the device to a repair shop.

You can also test a capacitor by powering on the device and measuring voltage across its leads with the meter set to voltage mode. A working capacitor will show a voltage that changes over time as it charges and discharges. A failed capacitor will show zero or a steady voltage that does not change. This method is riskier because the device is powered on, so use it only if you are comfortable working with live circuits.

Replacing a failed capacitor

If your test shows the capacitor has failed, you will need to replace it. Buy a replacement with the same capacitance value and voltage rating — or a higher voltage rating if the exact match is not available. Solder the old capacitor out and the new one in, making sure the polarity is correct (positive lead to positive, negative to negative) if it is an electrolytic capacitor.

If you are not comfortable soldering, take the device to a repair shop. Capacitor replacement is one of the cheapest repairs, usually costing $20 to $50 in labor plus the cost of the part.

Frequently Asked Questions

Can I test a capacitor while it is still soldered to the circuit board?

Yes, but the reading may be inaccurate because other components on the board can interfere with the measurement. For the most reliable result, desolder the capacitor first. If you do test it in place, discharge it first and be aware that a low or zero reading might be caused by the circuit, not the capacitor itself.

What if my multimeter does not have a capacitance setting?

Older or very cheap multimeters may not measure capacitance. You can still test the capacitor by looking for visible damage, or you can buy a dedicated capacitor tester. Alternatively, you can measure resistance across the capacitor leads — a working capacitor will show a very high resistance that slowly increases as it charges, while a failed capacitor will show zero resistance.

Is it safe to test a capacitor in a powered-on device?

No. Always turn off and unplug the device first. Testing a live capacitor risks electric shock and can damage your meter. The only exception is if you are trained in live circuit testing and using proper safety equipment.

Why does my meter show a reading that keeps changing?

Some meters display a slowly changing number while the capacitor charges through the meter's internal circuit. This is normal. Wait a few seconds for the reading to stabilize, then note the final value. If the reading keeps climbing or dropping after 10 seconds, the capacitor may be leaking and should be replaced.

Can a capacitor test good on a meter but still be bad?

Yes, rarely. A capacitor can pass a capacitance test but fail under load (when current flows through it) or at high temperature. If you have tested the capacitor and it reads normal, but the device still does not work, the problem is likely elsewhere. Check other components, solder joints, and connections before assuming the capacitor is bad.