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Technical Guide

How to Test a Capacitor with a Multimeter

Suspect a bad capacitor? Whether it's an air conditioner, pool pump, or vintage radio, testing a capacitor is a simple skill. This guide covers the 3 best methods to determine if your capacitor is good, dead, or failing.

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CRITICAL SAFETY WARNING

Capacitors store electrical energy like a battery. They can shock you even if the device is unplugged.

  • ALWAYS turn off power to the equipment.
  • ALWAYS discharge the capacitor before touching terminals.
  • Ideally, create a "Discharge Tool" using a 20kΩ 5W resistor.
  • If using a screwdriver to short terminals, wear safety glasses and use an insulated handle (expect a spark!).

Step 1: The Visual Inspection

Before pulling out your tools, look at the capacitor. Often, a bad capacitor will "tell" you it failed visually. If you see any of these signs, stop testing and replace it immediately.

Bulging or Domed Top

Electrolytic capacitors have a flat metal top. If it is domed, curved, or puffed out, internal pressure has built up. It is dead.

Leaking Fluid

Oil or electrolyte crying from the terminals or bottom seals means the seal has failed. This fluid can be corrosive.

Burn Marks

Black soot or scorch marks on the terminals or the PCB underneath indicate catastrophic failure or arcing.

Separated Casing

If the plastic sleeve has shrunk back or the aluminum can has pushed away from the base, the capacitor has overheated.

Best Method

Method 2: Testing with Capacitance Mode

This is the most accurate way. You need a multimeter that has a specific capacitor testing function. Look for the symbol -||- on the dial.

1

Isolate

Disconnect the capacitor from the circuit. If it is soldered, you must desolder at least one leg.

2

Discharge

Short the terminals for a few seconds to drain any remaining voltage.

3

Set to Capacitance

Turn your multimeter dial to the Capacitor symbol (-||-) or the "nF / µF" setting.

4

Connect Probes

Connect red to positive (+), black to negative (-). polarity matters for electrolytic caps, but not for non-polarized (motor/film) caps.

5

Wait & Read

It may take a few seconds for the reading to stabilize. Compare this number to the label (or decode it with our Capacitor Codes Guide).

How to Interpret Results:

  • Good: Reading is within ±% tolerance of label (e.g. 100µF ±20% = 80-120µF). Not sure about the label? Check our Codes Guide.
  • Weak: Reading is below tolerance (e.g. 70µF on a 100µF cap). It works but efficiency suffers.
  • Dead: Reading is "OL" (Over Limit) immediately or zero.

Method 3: Testing with Resistance (Ohms)

Vintage Tech Tip: The Analog Advantage

Old-school analog (needle) multimeters are actually better for this specific test. Watching the needle swing right (low resistance) and slowly drift back left (charging) gives a much clearer visual of the capacitor's health than dancing numbers on a digital screen.

Use this if your multimeter does not have a capacitor mode. It won't give you the µF value, but it can tell you if the capacitor is completely dead (shorted or open).

1. Set to High Resistance

Turn dial to Ohms (Ω) range. Use 20kΩ for small caps, 1MΩ for large motor caps.

2. Connect Probes

Touch probes to terminals. Polarity doesn't matter for this pass/fail test, but red-to-positive is good practice.

3. Interpret Behavior

GOOD Behavior

Resistance starts low (near 0Ω) and climbs steadily up. Eventually, it hits "OL" (Infinity).

Why? The battery in your meter is charging the capacitor. As it fills up, it resists more current.

BAD Behavior
  • Stays at 0Ω: Shorted internally. (Dead)
  • Stays at OL immediately: Open circuit. (Dead)
  • Stays at a fixed number (e.g. 50kΩ): Leaking heavily. (Failing)

Advanced Concept: Dielectric Absorption

Ever discharged a capacitor, come back 10 minutes later, and got shocked? That's Dielectric Absorption (or "Memory Effect"). The dielectric material retains a charge deep in its molecular structure that slowly releases. Always leave a discharge resistor connected if you plan to leave high-voltage caps sitting on a bench.

Frequently Asked Questions

How do I know if a capacitor is bad without a multimeter?
Visual signs are the best indicator without tools. Look for: 1) Bulging or domed top (on electrolytic cans), 2) Leaking oil or electrolyte fluid, 3) Burnt smell or scorch marks, 4) The casing has popped off or cracked. However, a capacitor can look perfect but still be electrically "open" or "weak," so a multimeter test is the only specific way to know for sure.
How do you test a capacitor with a multimeter without capacitance mode?
Use resistance mode (ohms). First discharge and isolate the capacitor. Place probes on the terminals: a good capacitor shows a low reading that rises toward OL as it charges. A constant low reading suggests shorted; immediate OL without movement suggests open.
What setting do I use on my multimeter?
Use the Capacitance setting, typically marked with the symbol -||- or sometimes "nF" / "µF". If your meter does not have this mode, you can use the Resistance (Ohms / Ω) setting for a rough "pass/fail" test, but it won't tell you the capacitance value.
Can I test a capacitor while it is still in the circuit?
No. You must disconnect at least one leg of the capacitor from the circuit to get an accurate reading. Other components in the circuit (like motor windings or resistors) will interfere with the multimeter's signal, giving you false results.
What should a good capacitor read?
In Capacitance mode, it should read close to the value printed on the label (e.g., a 40µF capacitor should read between 38µF and 42µF, assuming a ±5% tolerance). In Resistance mode, it should start at a low resistance and climb steadily toward infinity (OL) as it charges.
How do I discharge a capacitor safely?
Use a high-wattage resistor (like 20kΩ 5W) and hold it across the terminals with insulated pliers for 5-10 seconds. For small low-voltage capacitors, you can use an insulated screwdriver to short the terminals, but this creates a spark and can damage the terminals, so a resistor is preferred.
My multimeter reads "OL". Is the capacitor bad?
It depends on the mode. In Resistance (Ohms) mode, "OL" (Over Limit or Infinity) after the numbers climbed up means the capacitor is GOOD (it is holding a charge). If it stays at "OL" immediately without climbing, it is OPEN (bad). In Capacitance mode, "OL" usually means the capacitor is shorted or the value is out of the meter's range.

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