What a capacitor test tells you
A capacitor is a component that stores electrical charge temporarily. When it fails, it can cause a device to stop working, display flickering, or behave erratically. Testing a capacitor with a multimeter tells you whether it is still holding a charge the way it should, or whether it has failed and needs replacement.
A multimeter can test a capacitor in two ways: by measuring resistance (which changes as the capacitor charges and discharges) or by using a dedicated capacitance setting if your multimeter has one. The resistance method works on most multimeters. The capacitance method is faster and more direct, but not all multimeters include it.
Before you test, you must discharge the capacitor safely. A charged capacitor can deliver a shock even when the device is unplugged. Discharging takes 30 seconds and prevents injury.
Key Takeaways
- Always discharge a capacitor before testing by touching a screwdriver across both leads to release stored charge safely.
- If your multimeter has a capacitance setting (marked µF or nF), switch to it, touch the probes to the capacitor leads, and read the number directly.
- If your multimeter only has resistance settings, switch to the highest ohm range, touch the probes to the leads, and watch the needle or number rise gradually — a rising reading means the capacitor is good.
- A capacitor that shows zero resistance, stays at zero, or reads infinity on a resistance test has failed and should be replaced.
Discharging the capacitor safely
Before you touch a capacitor with anything, discharge it. Even a small capacitor can hold enough charge to cause pain or damage. The discharge takes seconds and is the most important step.
Locate an insulated screwdriver or a wire with an insulated handle. Touch the metal tip or wire across both leads of the capacitor at the same time. You may see a small spark or hear a tiny pop — this is normal and means the charge released. If you see nothing, the capacitor was already discharged or is dead.
After discharging, you can safely handle the capacitor and test it. Do not skip this step even if the device has been unplugged for hours.
Testing with a capacitance setting
If your multimeter has a dial or menu with a capacitance setting, this is the fastest method. The setting is usually marked with the symbol µF (microfarads) or nF (nanofarads), sometimes with a picture of two parallel lines.
Set the dial or menu to capacitance. If your multimeter has multiple capacitance ranges (like 20µF, 200µF, 2000µF), start with the range closest to the capacitor's rated value — you will find this number printed on the capacitor itself. If you are unsure, start with the highest range and work down.
Touch the red probe to the positive lead of the capacitor and the black probe to the negative lead. The multimeter will display a number. A reading close to the rated value (within about 20 percent) means the capacitor is good. A reading of zero or a reading far below the rated value means the capacitor has failed.
Testing with a resistance setting
Most multimeters have a resistance setting even if they lack capacitance. This method is slower but works on nearly every multimeter. Set the dial to the highest ohm range — usually marked 20M or 2M (meaning 20 million or 2 million ohms).
Touch the red probe to the positive lead and the black probe to the negative lead. Watch the display or needle. A good capacitor will show a high resistance reading that gradually rises toward infinity over a few seconds. This rising reading happens because the capacitor is charging through the multimeter's internal battery. The gradual climb is the sign of a healthy capacitor.
If the reading stays at zero, jumps when ready to infinity without rising, or does not change at all, the capacitor has failed. A capacitor that shows zero resistance is shorted internally. One that shows infinity when ready is open internally. Either way, it needs replacement.
What the readings mean
A capacitor can fail in two main ways: it can short (resistance drops to zero) or it can open (resistance stays at infinity). A shorted capacitor allows current to flow freely when it should block it, which can damage other components. An open capacitor blocks all current, which stops the circuit from working.
On a resistance test, a good capacitor shows a gradual climb from low resistance to high resistance. This happens because the capacitor charges through the multimeter's internal battery. The climb takes a few seconds. If you see this pattern, the capacitor is good.
If you test the same capacitor twice in a row without discharging it between tests, the second test may show different results because the capacitor is already charged. Always discharge between tests.
Capacitors that are hard to test
Electrolytic capacitors (the cylindrical ones with leads) are the easiest to test because they are polarized — the positive and negative leads are marked. Ceramic and film capacitors (the small flat ones) have no polarity, so you can touch the probes either way.
Some capacitors are soldered directly to a circuit board and cannot be tested in place. If the capacitor is still attached to the board, the resistance of the rest of the circuit will interfere with your reading. You must desolder the capacitor first, test it, and then decide whether to replace it.
Very large capacitors (above 10,000 µF) charge slowly and may take 10 to 20 seconds to show a full reading on a resistance test. Be patient and wait for the reading to stabilize before deciding whether it is good or bad.
When to replace instead of test
If a device stopped working and a capacitor looks visibly damaged — bulging at the top, leaking fluid, or burned — replace it without testing. Visual damage is a reliable sign of failure. Testing a visibly bad capacitor wastes time.
If you are not certain which capacitor caused the problem, test the ones that are most likely to fail first. Electrolytic capacitors fail more often than ceramic ones. Capacitors near heat sources (next to a transformer or power supply) fail sooner than ones in cooler areas.
If testing shows the capacitor is good but the device still does not work, the problem is elsewhere. Move on to testing other components or checking connections.
Frequently Asked Questions
Can I test a capacitor while it is still soldered to the board?
Not reliably. The rest of the circuit provides an alternate path for current, which throws off the reading. If the capacitor is the suspected problem, desolder it first. If you are not comfortable desoldering, take the device to a repair technician.
What if the capacitor reads lower than its rated value but not zero?
A capacitor that reads 30 to 50 percent below its rated value is degrading and should be replaced soon, even if the device still works. Degraded capacitors fail suddenly and often take other components with them.
Do I need to discharge a capacitor if the device has been unplugged for a week?
Yes. Large capacitors can hold a charge for days or weeks. Discharge it anyway — the 30 seconds it takes is worth avoiding a shock. Do not assume time alone has discharged it.
Why does the resistance reading climb slowly instead of jumping to a number?
The multimeter's internal battery is charging the capacitor through the probes. As the capacitor charges, its resistance increases. The climb is normal and is actually the sign of a good capacitor. A bad capacitor will not climb at all.
Can a capacitor test good but still be bad?
Rarely. A capacitor that passes a resistance or capacitance test is almost certainly functional. If the device still does not work after testing the capacitor, the problem is a different component or a broken connection.