Test a Capacitor with a Multimeter
Reading microfarads against the label, after the charge is gone.
Compiled by DIY Repair Guide · reviewed 2026-09-14 ·how we research these
To test a capacitor with a multimeter, switch off all power to the equipment at the breaker panel and the outdoor disconnect box. Confirm with your meter that power is off, discharge the capacitor with a resistor, check again for zero volts, then remove the wires. Turn your multimeter dial to capacitance mode, place the test probes across the terminals, and compare the reading in microfarads (µF) to the rated value printed on the part label.
Capacitors in an air conditioner or heating, ventilation and air conditioning (HVAC) system store high voltage. A good capacitor can hold that charge after you disconnect power from the unit. Touching energized terminals can cause a severe electrical shock.
Safety First and Stored Charge
Before touching any capacitor, you must disconnect all circuit power and safely discharge the stored electrical charge across the terminals. A good capacitor stores an electrical charge and can remain energized long after power is disconnected. According to the Fluke 15B and 17B Users Manual, voltages exceeding 30 volts alternating current (AC) root-mean-square, 42 volts AC peak, or 60 volts direct current pose an electrical shock hazard. Air conditioner condensers operate on 240 volts AC, meaning their capacitors can hold hazardous energy.
Capacitor manufacturer Genteq explains that simply turning off the cooling equipment at the indoor thermostat is insufficient for safety before handling a capacitor. Switch off the condenser's breaker and the outdoor disconnect, then use your meter's AC voltage setting to confirm the power is off.
Published guidance from Fluke specifies connecting a 20,000-ohm, 5-watt resistor across the capacitor terminals for five seconds to discharge the component safely. On a dual run capacitor, discharge and check both HERM to C and FAN to C, using your meter's DC voltage setting to confirm each reads zero. Never short the terminals with a screwdriver or other metal tool. If you do not have the proper resistor or feel uncomfortable working around high-voltage equipment, stop and call a licensed HVAC technician.
Never touch capacitor terminals without confirming zero voltage on your meter. Always switch off the breaker, switch off the outdoor disconnect, and bleed remaining charge with a 20,000-ohm, 5-watt resistor held across the terminals for five seconds.
Reading the Capacitor Label
A capacitor label lists its capacitance rating, working voltage, and terminal configuration. Capacitance is measured in microfarads (µF), though older or alternative markings on the metal can may print MFD or MF. Manufacturer Genteq notes that ratings also include a voltage rating in volts AC (VAC), typically 370 V or 440 V, and often a temperature rating and safety compliance markings. When replacing a unit, the voltage rating of the new part must equal or exceed the original rating, while the capacitance rating must match the equipment specification exactly.
Outdoor air conditioner units often use a dual run capacitor that combines two capacitors inside one metal cylinder. Genteq explains that run capacitors stay connected in the circuit continuously while the motor runs, unlike start capacitors that disconnect via a relay after startup. A dual run capacitor features three sets of terminal posts on top: HERM for the hermetic compressor, FAN for the outdoor condenser fan motor, and C for the shared common connection.
Testing in Capacitance Mode
An accurate capacitor test needs a multimeter with a capacitance mode. To set up your meter, turn the rotary dial to the capacitance setting, marked with the letter F for farads or the standard capacitor symbol. As noted in Fluke dial and symbol guides, this position is sometimes shared with another function, requiring you to press a secondary function button to activate the farad reading. You can review dial markings in detail on our guide to capacitance and other dial symbols.
Disconnect the wire leads from the capacitor terminals before testing. Fluke says to detach the capacitor from the circuit to avoid measurement errors. Fluke explains that a multimeter determines capacitance by charging the component with a known current, measuring the resulting voltage, and calculating the capacitance value.
Connect your black meter probe to the C terminal and your red meter probe to the HERM terminal. Keep the probes firmly connected for a few seconds. The Fluke 15B and 17B Users Manual notes that capacitance readings can take up to 15 seconds to stabilize because the meter charges the component to calculate the value. For low readings in nanofarads (nF), Fluke recommends activating Relative mode to zero out test lead capacitance. For a dual run capacitor, repeat the test between the C and FAN terminals.
Interpreting Your Multimeter Reading
Compare your multimeter reading to the microfarad rating printed on the capacitor label. Capacitor manufacturer Genteq states that standard capacitance tolerance is typically ±6 percent, and any measurement falling below 80 percent of the rated capacity indicates that replacement is necessary. A reading between those two lines is outside Genteq's typical tolerance but above its replacement threshold, so have a licensed HVAC technician check the unit.
If your meter screen shows the over limit (OL) indicator, the capacitor is either faulty due to an open circuit or possesses a capacitance value higher than your meter's maximum range. Fluke notes that faulty capacitors can develop short circuits, open circuits, or physical deterioration that alters their internal capacitance.
Consider a hypothetical dual run capacitor rated at 45/5 µF with a ±6 percent tolerance. For the 45 µF compressor section, 6 percent equals 2.7 µF, yielding an acceptable operational window of 42.3 µF to 47.7 µF. If that section drops below 80 percent, measuring under 36 µF, it indicates that replacement is needed. For the 5 µF fan section, a 6 percent variation equals 0.3 µF, defining a normal window of 4.7 µF to 5.3 µF, with an 80 percent threshold at 4 µF. Because both sections share one housing, a reading below 80 percent on either section means replacing the whole capacitor.
| Reading | Diagnostic Status | Source Reference |
|---|---|---|
| Within ±6% of label rating | Normal operating range | Genteq typical specification |
| Below 80% of label rating | Worn component requiring replacement | Genteq manufacturer guidance |
| Displays OL (Over Limit) | Open circuit, faulty capacitor, or exceeds meter range | Fluke capacitance testing guide |
| Zero or near zero µF | Short circuit or complete internal failure | Fluke and Keysight documentation |
Multimeter Capacitance Measurement Ranges
A digital multimeter can only measure capacitors that fall within its specified measurement limits. The Fluke 15B and 17B digital multimeters feature five capacitance ranges spanning from 50.00 nF up to a maximum limit of 100.0 µF, with an accuracy rating of 5 percent plus 5 counts on microfarad scales. If you attempt to test a large industrial capacitor rated above 100 µF with that specific meter, the display will show OL even if the component is fully functional.
Check your multimeter user manual to confirm its maximum farad rating before diagnosing cooling equipment. You can consult our reference on how to use a multimeter for instructions on setting test jacks and function buttons correctly.
Testing Without Capacitance Mode
If your multimeter lacks a capacitance setting, you can perform a basic screening test using the resistance mode. Test equipment manufacturer Keysight explains that after disconnecting and safely discharging the capacitor, setting your meter to measure ohms provides a functional check. When you place meter probes across the terminals, a healthy capacitor shows a low resistance at first that gradually rises as it charges.
Keysight notes that a faulty capacitor will show constant low resistance if shorted internally, or continuous open circuit resistance without any shifting values. This resistance test serves only as a rough go or no-go screen. It cannot measure microfarads, cannot verify if a component is within its ±6 percent tolerance, and cannot confirm whether the capacitor has degraded below the 80 percent threshold. Use this method only to identify dead shorts until you can verify the unit with a capacitance-capable meter.
Visual Signs of Capacitor Failure
Physical inspection can confirm capacitor failure before you take an electrical measurement. Both Genteq and Keysight state that bulging or swelling of the cylindrical casing and leaked dielectric fluid are physical signs of a failed capacitor. Other visible defects include scorched burn marks around terminal spade connectors, charred wiring insulation, and corrosion on the metal seams.
System symptoms also suggest a failing run capacitor. Genteq notes that starting failures are the most common operational symptom, occurring when an air conditioning system fails to start when the thermostat calls for cooling. If your outdoor compressor motor hums loudly without turning over, Genteq says the motor is energized but cannot generate enough starting torque. For a complete system checklist, see our guide on troubleshooting an AC that is not cooling.
When to Call an HVAC Technician
Stop and call a licensed HVAC technician if any of the conditions listed below apply.
When a technician or qualified person replaces the capacitor, manufacturer Genteq specifies that the replacement component must match the original microfarad rating exactly. The voltage rating must meet or exceed the original specification, such as installing a 440 VAC capacitor in place of a 370 VAC part, but never a lower voltage rating.
- Missing measurement tools: You do not own a multimeter equipped with capacitance mode or lack the 20,000-ohm, 5-watt resistor needed to discharge the terminals safely.
- Visible component damage: The capacitor casing shows bulging, swelling, leaking dielectric fluid, or burnt terminal connections.
- Motor failure symptoms: The compressor hums but will not start.
- High-voltage discomfort: You feel uncertain about pulling disconnects or taking live voltage verification readings inside a 240-volt enclosure.
Common questions
What is the symbol on a multimeter for testing a capacitor?
Look for the capacitance position marked with the capital letter F for farads or the capacitor symbol. On many digital meters, this setting shares dial space with resistance or continuity and requires pressing a function button to activate.
Can you test a capacitor with a regular meter?
Yes, once the capacitor is discharged, you can perform a basic go or no-go test using resistance mode if your meter lacks capacitance testing. Test equipment maker Keysight notes that a functional capacitor will show low resistance that gradually rises as it charges, while a faulty one shows no change. However, this test cannot measure microfarads or verify tolerance.
What multimeter to test capacitor?
You need a digital multimeter with a dedicated capacitance function whose upper range exceeds the microfarad rating printed on your capacitor label. For example, the Fluke 15B and 17B top out at 100.0 µF, so a capacitor rated above that reads OL on those meters.
What are the two typical signs that a capacitor is bad or has failed?
According to manufacturers Genteq and Keysight, common physical signs are a bulging or swollen casing and leaked dielectric fluid around the terminals or seams. Burn marks and terminal corrosion are also frequent secondary signs of failure.
Will the AC still run if the capacitor is bad?
An air conditioner often fails to run when a run capacitor degrades. As manufacturer Genteq explains, common symptoms include the system failing to respond to cooling calls or the compressor humming loudly without turning over because it cannot generate starting torque.