What you're actually testing when you check a capacitor

A capacitor is a component that stores electrical charge temporarily. When you check one, you're testing whether it can still hold that charge and release it properly. A bad capacitor won't store charge, will leak charge too fast, or will short out and damage the circuit around it. Most capacitors fail by drying out internally — the electrolyte inside evaporates over years of heat and use.

You can check a capacitor three ways: visually for obvious damage, with a multimeter to measure its capacitance, or by testing it in the circuit while power is on (only if you know what you're doing). For most people, a multimeter test catches the problem without needing to remove the part.

Key Takeaways

  • Bulging, leaking, or burnt-looking capacitors are dead and need replacement — you don't need a meter to know this.
  • A multimeter set to capacitance mode can test a capacitor after you disconnect it from the circuit and discharge it safely.
  • The reading should match the number printed on the capacitor's side, within the tolerance range listed (usually 10 to 20 percent).
  • If the meter shows zero or a value far below the rating, the capacitor has failed and should be replaced.
  • Always discharge a capacitor before touching it — it can hold enough charge to hurt you even after power is off.

Visual inspection: the fastest check

Before you plug in a meter, look at the capacitor itself. Capacitors are usually cylindrical cans with two leads sticking out the bottom, or flat rectangular blocks. On cylindrical ones, the top may have a small X-shaped score mark or a vent hole.

If the top is bulging outward, the capacitor has failed — internal pressure built up as the electrolyte broke down. If you see brown or black crusty material leaking from the bottom or sides, it has leaked. If the casing is charred or blackened, it overheated and shorted. Any of these means replacement. You don't need a meter; the capacitor is dead.

If the capacitor looks normal — smooth sides, no bulge, no leaks, no burn marks — move to the meter test to know for certain.

How to safely discharge a capacitor before testing

A capacitor can hold a charge even after you turn off power and unplug the device. That charge can deliver a shock strong enough to hurt. Before you touch the leads or test it, you must discharge it safely.

The safest method is to use an insulated screwdriver. Hold the handle, then touch the metal tip across both leads at once — this creates a path for the charge to flow out. You'll see a small spark; that's the charge leaving. Do this once, wait a few seconds, then do it again to be sure. If you're uncomfortable with this step, use a resistor discharge tool (a resistor soldered between two wires with insulated handles), which is slower and safer but takes longer.

Never use your bare fingers or a metal tool you're holding in your hand. Never skip this step on large capacitors in power supplies or audio equipment — the shock can be serious.

Testing with a multimeter: the standard method

A multimeter is a handheld meter that measures voltage, current, and resistance. Most also measure capacitance. You need one that has a capacitance setting — look for a symbol that looks like two parallel lines or the letters "F" with a number (farads are the unit of capacitance).

First, remove the capacitor from the circuit if possible. If you can't remove it, you can test it in place, but the circuit around it may affect the reading. Discharge the capacitor using the screwdriver method above. Set the multimeter dial to the capacitance setting. If your meter has multiple capacitance ranges (like 20µF, 200µF, 2000µF), start with the range closest to the capacitor's rating — the number printed on its side.

Touch the meter's red probe to the positive lead (marked with a stripe or + symbol) and the black probe to the negative lead. Wait two to three seconds for the reading to stabilize. The meter will show a number. Compare it to the rating printed on the capacitor.

Reading the result and understanding tolerance

The capacitor's side is printed with a number and a unit — for example, "100µF" (microfarads) or "47nF" (nanofarads). This is the rated capacitance. The meter should show a value close to this number.

Capacitors are not perfectly accurate. They come with a tolerance — a range of acceptable values. This is usually printed as a percentage, like ±10% or ±20%. If a 100µF capacitor has ±10% tolerance, any reading between 90µF and 110µF is normal. Check the capacitor's label for the tolerance range.

If the meter reads within the tolerance range, the capacitor is good. If it reads zero, shows "OL" (overload), or reads far below the rated value, the capacitor has failed. A reading much higher than the rating is also a sign of failure, though less common.

Testing a capacitor while it's still in the circuit

If removing the capacitor is difficult or risky, you can test it in place. The circuit around it may interfere with the reading, so this method is less reliable, but it works in many cases.

Power off the device completely. Discharge the capacitor using the screwdriver method. Set your multimeter to capacitance mode and touch the probes to the capacitor's leads. If the reading is close to the rating, the capacitor is likely good. If it reads zero or very low, the capacitor has probably failed.

If the in-circuit reading is unclear, remove the capacitor and test it alone. This takes more time but gives you a definite answer.

When to replace instead of test

If a capacitor is visibly damaged — bulging, leaking, or burnt — replace it when ready. Don't bother testing. If the meter shows it's bad, replace it. If you're unsure whether the meter reading is accurate and the capacitor is in an old device, consider replacing it anyway. Capacitors that are failing often cause the whole device to malfunction, and replacement is usually cheap compared to the cost of the device.

When you replace a capacitor, match the voltage rating and capacitance exactly. The voltage rating is printed on the side (like "25V" or "450V"). Using a capacitor with a lower voltage rating can cause it to fail when ready. Using one with a higher voltage rating is safe but may not fit in the space.

Frequently Asked Questions

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

Yes, rarely. A capacitor can fail intermittently — it works when cold but fails when hot, or it works at low voltage but fails at high voltage. A meter test at low voltage may miss this. If a device keeps failing and the capacitor tested okay, try replacing it anyway, especially if the device is old.

What's the difference between microfarads and nanofarads?

A microfarad (µF) is one millionth of a farad. A nanofarad (nF) is one billionth of a farad. So 1000 nanofarads equals 1 microfarad. Check the label on your capacitor to know which unit it uses, then set your meter to the matching range.

Do I need to remove a capacitor to test it?

Not always. Testing in place works if the circuit around it doesn't interfere. But if the reading is unclear or you want a definite answer, remove it. Removing it takes a few minutes with a soldering iron and gives you a clean reading.

What does it mean if the meter shows OL?

OL means overload — the capacitor's value is too high for the meter's current range. Switch to a higher capacitance range on your meter and test again. If all ranges show OL, the capacitor may have failed or the meter doesn't go high enough for that capacitor's size.

Can I test a capacitor without discharging it first?

You can test it, but you risk a shock. Discharge it first — it takes 10 seconds and keeps you safe. On small capacitors in low-voltage circuits, the risk is low, but on large capacitors in power supplies, the shock can be serious.