What a capacitor does and why it fails
A capacitor is a small cylindrical or rectangular component that stores electrical energy and releases it in a burst. In motors — whether in an air conditioner, refrigerator, washing machine, or ceiling fan — the capacitor gives the motor the extra push it needs to start spinning and keeps it running smoothly. When a capacitor fails, the motor either won't start at all or runs but makes a humming sound without turning.
Capacitors fail because they age. The chemical inside breaks down over time, especially if the motor runs hot or the appliance sits unused for months. A failed capacitor is one of the most common reasons a motor stops working, and it's also one of the cheapest and easiest parts to replace — but only if you know how to check it first.
Before you buy a replacement or call a repair person, you can test the capacitor yourself with a multimeter. This takes about five minutes and tells you whether the capacitor is dead, weak, or still working.
Key Takeaways
- A capacitor that has failed will cause a motor to hum without spinning, fail to start, or stop working entirely.
- You can test a capacitor with a multimeter set to the resistance or capacitance setting, depending on what your meter can measure.
- Before testing, you must turn off the power to the appliance and discharge the capacitor by shorting its terminals with an insulated screwdriver to avoid electric shock.
- A capacitor reading near zero or showing no change when you test it is almost certainly failed and needs replacement.
- If the capacitor tests weak but not dead, the motor may still run but will draw more power and fail sooner than it should.
Gather the right tools and safety equipment
You need a multimeter — a handheld meter that measures voltage, resistance, and sometimes capacitance. A basic digital multimeter costs $15 to $30 and works for this job. If your multimeter has a capacitance setting (marked with a symbol that looks like two parallel lines), that's ideal. If not, you can still test with the resistance setting, though the result is less precise.
You also need an insulated screwdriver — one with a plastic handle — to discharge the capacitor before you touch it. A capacitor holds an electrical charge even when the power is off, and that charge can shock you or damage your meter. Discharging it safely is the most important step.
Wear safety glasses if you have them. A failed capacitor can occasionally leak or rupture, and you don't want that liquid in your eyes.
Turn off power and discharge the capacitor safely
Unplug the appliance or turn off the circuit breaker that powers it. Wait at least five minutes. This gives any stored charge time to leak away on its own, though you should not rely on this alone.
Locate the capacitor. It's usually a cylinder about the size of a AA battery, mounted on the motor or nearby. It will have two or three metal terminals sticking out of the top. Look at the label on the capacitor — it will show a voltage rating (usually 370V or 440V) and a capacitance value in microfarads, written as µF or MFD.
Take your insulated screwdriver and touch both terminals at the same time, holding the blade across them for a second or two. You may see a small spark — that's the stored charge leaving the capacitor, and it's normal. This step is essential. Do not skip it, even if you think the capacitor is dead.
Test with a multimeter on the capacitance setting
If your multimeter has a capacitance setting, this is the most straightforward test. Set the dial to the capacitance range. Most meters have a setting marked µF or nF (nanofarads). Start with the highest range if you're not sure.
Touch the red probe to one terminal and the black probe to the other. The meter will display a number. A working capacitor will show a reading close to the value printed on the label — for example, if the label says 45 µF, a healthy capacitor should read somewhere between 40 and 50 µF. Some variation is normal; anything within 10 percent of the label value is acceptable.
If the meter reads zero or shows no change when you touch the probes, the capacitor is dead. If it reads significantly lower than the label — say, 20 µF when it should be 45 µF — the capacitor is weak and will fail soon. Replace it.
Test with a multimeter on the resistance setting if capacitance is not available
If your multimeter does not have a capacitance setting, switch to the resistance mode, usually marked with an Ω (ohm) symbol. Set it to the highest resistance range available.
Touch the red probe to one terminal and the black probe to the other. When you first make contact, the meter will jump to a low resistance reading, then climb back toward infinity (usually shown as "1" on the display or an open line). This climbing motion means the capacitor is charging through the meter's internal battery, which is what you want to see.
If the meter stays at zero resistance or shows no movement at all, the capacitor is shorted and dead. If the meter climbs partway and stops, the capacitor is leaking and weak. A good capacitor will climb all the way to infinity or very close to it.
This test is less precise than the capacitance method, but it tells you whether the capacitor is completely failed or still holding a charge.
Understand what the results mean for your motor
A capacitor that reads at or near the label value is working. Your motor should start and run normally. If the motor is still not working, the problem is elsewhere — a bad motor winding, a broken switch, or a wiring issue.
A capacitor that reads significantly lower than the label value or shows no movement is weak or dead. The motor may hum without starting, start slowly, or run but overheat. Replace the capacitor with one that has the same voltage rating and microfarad value. The label on the old capacitor tells you exactly what to buy.
A capacitor that reads zero or shows a short is completely failed. Replace it when ready. A dead capacitor will not let the motor start at all, and leaving it in place can damage the motor itself over time.
Know when to replace versus when to call a technician
If your test shows the capacitor is failed, you can buy a replacement at an appliance parts store, hardware store, or online. Bring the old capacitor or write down the voltage and microfarad rating. Installation is usually straightforward — disconnect the old one by unbolting or unclipping it, and bolt or clip the new one in the same position, matching the terminals.
If your test shows the capacitor is good but the motor still does not work, or if you are uncomfortable working with electrical components, call a repair technician. A failed capacitor is cheap and straightforward to replace, but other motor problems require professional diagnosis.
If the capacitor is visibly swollen, leaking, or has a burn mark, do not test it — replace it. These are signs of internal failure, and the capacitor is unsafe to use.
Frequently Asked Questions
Can a capacitor fail without any warning?
Yes. A capacitor can fail suddenly, especially if the motor ran hot or the appliance was stored in a humid place. You may notice the motor humming but not turning, or it may straightforward stop working one day. Testing the capacitor is the fastest way to rule it in or out as the cause.
Is it dangerous to test a capacitor myself?
It is safe if you discharge the capacitor first by shorting the terminals with an insulated screwdriver. The discharge step removes the electrical charge that could shock you. Always do this before touching the capacitor or connecting your meter.
What if my multimeter does not have a capacitance setting?
You can still test with the resistance setting. The meter will show whether the capacitor is holding a charge and climbing toward infinity, which tells you if it is working. This method is less precise but still useful for identifying a dead capacitor.
Can I use the same capacitor in a different appliance?
Only if the voltage rating and microfarad value match exactly. A capacitor rated for 370V will not work safely in a motor that needs 440V. Always match both numbers on the label.
How long does a capacitor usually last?
Most motor capacitors last 10 to 20 years, depending on how often the motor runs and how hot it gets. Capacitors in air conditioners and refrigerators, which run constantly in hot conditions, fail sooner than those in ceiling fans or washing machines.