What a multimeter tells you about a capacitor

A capacitor is a component that stores electrical charge. When it fails, it can cause a device to stop working or behave erratically. A multimeter — a handheld meter that measures voltage, resistance, and other electrical properties — can tell you whether a capacitor is dead, leaking, or still functioning.

The most reliable test uses the multimeter's resistance or capacitance setting. A good capacitor will show a specific pattern: the resistance reading will start low and climb toward infinity as the capacitor charges. A failed capacitor will either stay at zero resistance, jump straight to infinity, or show no change at all. This article walks you through the exact steps to perform this test safely.

Key Takeaways

  • Always power off and unplug the device before testing any capacitor inside it.
  • Discharge the capacitor by touching a screwdriver across its leads to release any stored charge that could damage the multimeter or shock you.
  • Set your multimeter to the resistance (ohms) setting, usually marked with the Greek letter omega (Ω), or use the capacitance setting if your meter has one.
  • A working capacitor will show resistance climbing from near zero toward infinity; a failed one will show zero, infinity, or no movement.
  • If the capacitor is soldered to a circuit board, you may need to desolder one lead first to get an accurate reading.

Gather your tools and prepare the device

You will need a multimeter, a screwdriver (preferably with an insulated handle), and the device containing the capacitor. Before you touch anything inside, unplug the device from the wall outlet and wait at least one minute. Even after unplugging, capacitors can hold a charge that will damage a multimeter or cause a painful shock.

Open the device according to its design — this might mean removing screws from the back, prying off a plastic cover, or unclipping a battery compartment. Once you can see the circuit board, locate the capacitor you want to test. Capacitors are usually cylindrical (like a small can) or rectangular, and they have two metal leads or terminals sticking out from the bottom or sides.

Discharge the capacitor safely

Before you connect the multimeter, you must discharge any charge stored in the capacitor. Take an insulated screwdriver and touch both leads of the capacitor at the same time for one to two seconds. You may see a small spark — this is normal and means the capacitor was holding charge. If there is no spark, the capacitor may already be dead or fully discharged.

Discharging prevents two problems: it protects the multimeter from a voltage spike that could damage its internal circuits, and it protects you from a shock. This step takes ten seconds and is not optional.

Set the multimeter to resistance mode

Turn the multimeter's dial to the resistance setting. On most meters, this is marked with the Greek letter omega (Ω) and is usually red or in a dedicated section of the dial. If your multimeter has a capacitance setting — marked with a capacitor symbol or the letters "F" or "µF" — you can use that instead, but resistance mode works on almost any meter.

If you are using resistance mode and your meter has multiple ohm ranges (like 200Ω, 2kΩ, 20kΩ), start with the highest range. This protects the meter if the capacitor is shorted.

Connect the multimeter leads and read the result

Touch the multimeter's red probe to the positive lead of the capacitor and the black probe to the negative lead. On a cylindrical capacitor, the positive lead is usually longer or marked with a plus sign. On a rectangular capacitor, look for a plus or minus symbol printed on the case.

Watch the display. A working capacitor will show a low resistance reading (close to zero) that slowly climbs toward infinity over two to five seconds. This climbing happens because the capacitor is charging through the multimeter's internal circuit. A failed capacitor will show one of three patterns: it will stay at zero ohms (shorted), jump instantly to infinity (open circuit), or show no change at all.

If you get a reading that does not fit these patterns, or if you are unsure, try the test a second time. Reverse the probe positions — touch red to negative and black to positive — and watch again. The result should be similar.

What to do if the capacitor is soldered to the board

If the capacitor is soldered directly to the circuit board and you cannot remove it by hand, you have two choices. The first is to desolder one lead using a soldering iron and solder wick or a desoldering pump. This removes the capacitor from the circuit so the multimeter measures only the capacitor, not the rest of the board. The second is to accept that the reading may be affected by other components on the board and use it only as a rough indicator.

If you choose to desolder, heat the solder joint for three to five seconds with the soldering iron, then use the wick or pump to remove the melted solder. Let the joint cool before touching it. Once one lead is free, you can perform the test as described above. After testing, you can resolder the lead back in place or replace the capacitor if it failed.

Interpreting unusual readings

Sometimes a capacitor will show a reading that does not clearly indicate pass or fail. A capacitor that climbs very slowly or only partway toward infinity may be degraded but still partially functional — it is not as reliable as one that climbs smoothly and completely. A capacitor that shows a very high resistance from the start (close to infinity) but does not climb further may be open internally, meaning it cannot hold a charge.

If you are testing a large capacitor (above 10 microfarads), the climb to infinity may take longer than ten seconds. Be patient and watch for at least fifteen seconds before concluding the test. If you are unsure whether a capacitor has failed, the safest approach is to replace it, especially if the device is not working correctly and the capacitor is a known failure point for that model.

Frequently Asked Questions

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

Yes, but the reading will be less reliable because other components on the board affect the resistance measurement. If the test shows the capacitor is clearly failed (stuck at zero or infinity), you can trust that result. If the reading is unclear, desolder one lead and test again for a definitive answer.

What if the multimeter display does not change at all?

A capacitor that shows no change in resistance — staying at the same number for ten seconds — is usually failed. It may be open (broken internally) or shorted. Try reversing the probe positions and testing again. If there is still no change, the capacitor should be replaced.

Do I need a special multimeter to test capacitors?

No. Any multimeter with a resistance (ohms) setting can test a capacitor using the climbing-resistance method described here. A meter with a dedicated capacitance setting is more convenient and gives a direct reading in microfarads, but it is not required.

Can testing a capacitor damage my multimeter?

If you discharge the capacitor first and use the correct meter setting, testing is safe. The main risk is testing a charged capacitor without discharging it first — the voltage spike can damage the meter's input circuits. Always discharge before connecting the probes.

How do I know which lead is positive on a capacitor?

On cylindrical capacitors, the longer lead is positive. On rectangular capacitors, look for a plus sign or a stripe printed on the case near the positive lead. If you cannot see a marking, test both ways — the result should be similar either way, though the climb may be faster in one direction.