What a motor capacitor does and why it fails
A motor capacitor stores electrical charge and releases it to help start or run an electric motor. Over time, capacitors lose their ability to hold a charge — usually after five to ten years, or sooner if the motor runs hot or the capacitor was defective from the start. A failed capacitor will not start the motor, or the motor will run but sound strained or overheat.
You can test whether a capacitor is dead or still working using a multimeter, a handheld tool that measures voltage, resistance, and other electrical properties. The test takes a few minutes and tells you whether the capacitor needs replacement. A multimeter costs between $15 and $50 for a basic model.
Key Takeaways
- Turn off power to the motor and discharge the capacitor by touching a screwdriver across its terminals before you touch it with your hands.
- Set your multimeter to the resistance setting (ohms) and touch the probes to each terminal — a working capacitor will show a rising resistance reading that climbs toward infinity.
- If the resistance stays at zero or does not climb, the capacitor is shorted and needs replacement.
- If the resistance climbs but the motor still will not start, the capacitor may be weak, and you should test it again or replace it as a precaution.
How to safely discharge a capacitor before testing
A capacitor holds an electrical charge even after you turn off the motor. Touching the terminals with bare hands can shock you. Before you touch the capacitor, you must discharge it safely.
Turn off power to the motor at the breaker or switch. Wait at least five minutes. Take an insulated screwdriver — one with a plastic handle — and touch the metal shaft to both terminals of the capacitor at the same time. You may see a small spark; that is the charge leaving the capacitor. Touch the terminals again to make sure no charge remains. Now the capacitor is safe to handle.
Setting up your multimeter for the resistance test
A multimeter has a dial or buttons that let you choose what to measure. You need the resistance setting, marked with the Greek letter omega (Ω) or the word "ohms". On an analog multimeter with a dial, turn the dial to the ohms setting — usually marked as R×1, R×10, or R×100. On a digital multimeter with buttons, press the button labeled "Ω" or look for a menu option for resistance.
If your multimeter has multiple ohms settings, start with R×1 or the lowest ohms range. You can switch to a higher range if the needle or display does not move. Do not worry about picking the exact right range — the test works on any ohms setting.
Testing the capacitor and reading the results
Touch one probe of the multimeter to one terminal of the capacitor and the other probe to the other terminal. Watch the needle or digital display. A working capacitor will show a resistance reading that starts low and climbs toward infinity (usually shown as "1" on a digital display or the needle moving all the way to the right on an analog meter). This climb happens because the capacitor is charging through the multimeter's internal battery.
If the resistance stays at zero and does not climb, the capacitor is shorted and has failed. If the resistance climbs partway but not all the way to infinity, the capacitor is weak. Reverse the probes and test again — you should see the same climbing pattern. If you see no movement either way, the capacitor is open (broken internally) and has failed.
What to do if the test shows the capacitor is bad
If the capacitor failed the test, it needs replacement. Write down the capacitor's size — printed on the side in microfarads (µF) and voltage (V), such as "10 µF 370V". Buy a replacement capacitor with the same or higher voltage rating and the same microfarad rating. Install it by disconnecting the old wires and connecting the new ones to the same terminals.
If the capacitor passed the test but the motor still will not start, the problem may be elsewhere — a bad switch, a broken motor winding, or a weak capacitor that is not quite dead yet. You can replace the capacitor anyway as a precaution, since capacitors are inexpensive and a weak one will fail soon.
When to call a repair technician instead
If you are not comfortable working with electrical equipment, or if the motor is part of an air conditioner, refrigerator, or other sealed system, do not open it yourself. Call a repair technician. Sealed systems require special tools and training, and opening them can release refrigerant or cause injury.
If you have tested the capacitor and it passed but the motor still does not work, a technician can test other parts of the motor circuit — the switch, the windings, and the power supply — to find the real problem.
Frequently Asked Questions
Can I test a capacitor while it is still connected to the motor?
No. Disconnect the capacitor from the motor first. Testing it while connected can give a false reading because other parts of the circuit are in the path. Always discharge it before you disconnect it.
What if my multimeter does not have an ohms setting?
Most multimeters have a resistance setting. If yours does not, you have a very basic model. Consider buying a standard multimeter for about $20 — it will measure voltage, resistance, and continuity, which covers most electrical troubleshooting.
Is a weak capacitor the same as a bad one?
A weak capacitor still holds some charge but not enough to start or run the motor reliably. It will eventually fail completely. If your test shows a weak capacitor, replacing it now prevents the motor from dying unexpectedly later.
Can a capacitor fail and then work again?
No. Once a capacitor fails, it stays failed. If your test shows it is bad today, it will not recover. If the motor works after you test it, the capacitor passed the test and is not the problem.