Testing an electrolytic capacitor takes just a few minutes with a standard digital multimeter, and you can catch most common failures without any specialized equipment. The process involves checking for physical damage, measuring capacitance (if your meter supports it), and using the resistance or continuity mode to identify shorted or open capacitors. For more precise diagnostics, an ESR meter or LCR meter can reveal problems a multimeter misses.
Discharge the Capacitor First
Before touching any capacitor terminals, you need to safely discharge any stored energy. Even a capacitor that’s been sitting on a shelf can hold a charge, and larger ones in power supplies or amplifiers can deliver a dangerous shock. The safe method is to connect a resistor across the two terminals and let the stored energy bleed off as heat. A 10kΩ resistor works well for most situations. For a rough sense of timing: a 1,000µF capacitor charged to 350V through a 10kΩ resistor has a time constant of about 10 seconds, meaning it takes roughly 50 seconds (five time constants) to fully discharge. Wait that full duration before handling anything.
Never short the terminals with a screwdriver or bare wire. That uncontrolled discharge can damage the capacitor, weld your tool to the contacts, or send sparks flying. Once you’ve discharged the cap, confirm it reads near zero volts with your multimeter set to DC voltage before proceeding.
Start With a Visual Inspection
Many failed electrolytic capacitors announce themselves before you pick up a meter. Look for a domed or bulging top. Aluminum electrolytic caps have scored vent lines on their tops specifically designed to release pressure during failure, so a raised or split top is a clear sign the cap is dead. Check the base of the capacitor for dried electrolyte residue, which often appears as a crusty brown or white deposit on the circuit board. If the safety vent has popped and ejected material, the capacitor has failed catastrophically and needs replacement, no further testing required.
Also look at the leads. Corroded or discolored legs suggest heat damage or electrolyte leakage that has attacked the metal over time. Any capacitor showing these physical signs should be replaced regardless of what your meter reads.
Testing With a Multimeter
Capacitance Mode
Many digital multimeters have a dedicated capacitance mode (often marked with a capacitor symbol or “F” on the dial). Remove the capacitor from the circuit, discharge it, then connect the positive (red) lead to the longer leg (the positive terminal) and the negative lead to the marked negative side. The meter will display a value in microfarads.
Compare the reading to the value printed on the capacitor’s label. Standard aluminum electrolytic capacitors have a tolerance of plus or minus 20%, so a 100µF cap reading anywhere from 80µF to 120µF is perfectly normal. Some electrolytics allow even wider tolerances, up to +80% above their rated value. A reading significantly below 80% of the labeled value means the capacitor has dried out and lost capacity. A reading of zero or near-zero means it’s open (the internal connection has broken).
Resistance (Ohm) Mode
If your multimeter lacks a capacitance mode, the resistance test can still catch the most common failures. Set your meter to a high resistance range (20kΩ or 200kΩ) and connect the leads to the capacitor terminals, observing polarity. On a healthy electrolytic capacitor, you should see the resistance start low and gradually climb toward a very high value as the meter’s internal battery charges the cap. This rising-resistance behavior is the hallmark of a working electrolytic.
If the meter reads zero ohms and stays there, the capacitor is shorted internally. If it immediately reads infinite resistance (OL or “open loop” on most meters) with no initial low reading, the capacitor is open. Either result means the cap is bad.
Why ESR Testing Matters
A capacitor can measure the correct capacitance on a multimeter and still cause problems in a circuit. The reason is equivalent series resistance, or ESR. This is the internal resistance of the capacitor that builds up as the electrolyte dries out over time. High ESR means the capacitor can’t deliver current fast enough for the circuit’s needs, even though it technically holds the right amount of charge. This is the single most common failure mode in aging electronics, especially power supplies and motherboards.
A standard multimeter cannot measure ESR. You need a dedicated ESR meter, which sends a small AC signal through the capacitor at a frequency (typically 100kHz) that reveals the internal resistance. Good electrolytic capacitors generally have ESR values well under 1 ohm for larger caps (hundreds of microfarads) and a few ohms for smaller ones. ESR meters come with reference charts showing acceptable ranges by capacitance value and voltage rating.
The biggest advantage of an ESR meter is that it works reliably with capacitors still soldered to the board. The low test voltage won’t forward-bias semiconductors in the surrounding circuit, so you get a meaningful reading without desoldering. This makes ESR meters extremely popular for repair work.
ESR Meter vs. LCR Meter
If you’re deciding what to buy, the choice comes down to how much testing you do and what components you work with. An ESR meter does one thing: it measures equivalent series resistance, and it does it in-circuit. It’s less expensive and straightforward to use, making it a solid starting point if you already own a multimeter.
An LCR meter measures capacitance, inductance, and resistance in one device. It offers higher accuracy by using both series and parallel measurement models depending on the component, and it can test a wider range of parts. If you own an LCR meter, you don’t need a separate ESR meter. But LCR meters cost more and typically require you to remove the component from the circuit for accurate readings.
For someone getting into electronics repair, a multimeter plus an ESR meter covers most capacitor diagnostics. If your work expands to include inductors, transformers, or precision capacitor selection, an LCR meter becomes worth the investment.
In-Circuit vs. Out-of-Circuit Testing
Testing a capacitor while it’s still soldered to a board saves time, but it introduces a real accuracy problem. Other components connected in parallel with the capacitor create alternate paths for your meter’s test signal. A resistor sitting across the same nodes might make a good capacitor look shorted, or a diode in the circuit might produce misleading resistance readings. ESR meters handle in-circuit testing better than multimeters because their high-frequency test signal is less affected by surrounding resistors and semiconductors.
If an in-circuit test gives you a suspicious result, desolder at least one leg of the capacitor and retest. Out-of-circuit measurements are always more reliable. For capacitance readings specifically, removing the component is the only way to get an accurate number.
What Causes Electrolytic Capacitors to Fail
Understanding why these capacitors fail helps you know what to test for. The primary killer is heat. Electrolytic capacitors contain a liquid or gel electrolyte that slowly evaporates through the rubber seal at the base. As electrolyte is lost, capacitance drops and ESR rises. This process follows a well-documented rule: for every 10°C increase in operating temperature, the capacitor’s lifespan roughly cuts in half. A cap rated for 2,000 hours at 105°C might last 16,000 hours at 75°C.
This is why capacitors near heat sinks, voltage regulators, or power transistors tend to fail first. Age alone will eventually get them too. A capacitor that’s 10 to 15 years old in a warm environment is a prime suspect regardless of how it looks externally. When troubleshooting older electronics, many repair technicians replace all the electrolytic caps in a hot zone rather than testing each one individually.
Leakage Current Testing
Leakage current is the small amount of DC current that flows through a capacitor even after it’s fully charged. All electrolytic capacitors have some leakage, but a failing cap leaks more. This test requires a bench power supply and a sensitive current meter (or a multimeter on its microamp range). You apply the capacitor’s rated voltage through a series resistor, wait for the charging current to settle (this can take anywhere from a few seconds to several minutes depending on the cap’s size), and then read the remaining current. That steady-state current is the leakage.
Acceptable leakage values vary by capacitor size and voltage rating, and manufacturers typically specify maximum leakage on their datasheets. This test is less common in everyday repair work but becomes important when you’re selecting capacitors for sensitive analog circuits or verifying new-old-stock components that have been sitting in storage for years. Capacitors that haven’t been powered in a long time sometimes need a slow “reforming” process, where you gradually bring them up to rated voltage, to restore the oxide layer and bring leakage current back to normal levels.