A starter capacitor stores electrical charge to help your motor start, and you can test whether it still holds that charge using a multimeter
A starter capacitor is a cylindrical component that gives your motor an extra electrical push when it first turns on. Over time, capacitors lose their ability to hold a charge — a condition called capacitance degradation. You can check whether yours is still working by using a multimeter set to measure resistance or capacitance, depending on what your meter can do. The test takes about five minutes and tells you whether the capacitor needs replacement.
The most common sign that a starter capacitor has failed is a motor that hums but does not turn, or one that turns only after you give it a manual spin. If your motor shows either symptom, testing the capacitor is usually the first step before you buy a replacement.
Key Takeaways
- Always disconnect power to the motor and discharge the capacitor by shorting its terminals with an insulated screwdriver before you touch it, because capacitors can hold a dangerous charge even when power is off.
- A multimeter set to the resistance setting (ohms) will show a rising resistance reading if the capacitor is good; a capacitor that reads zero or stays at zero has failed.
- If your multimeter has a dedicated capacitance setting (marked with a capacitor symbol), you can measure the microfarad (µF) rating directly and compare it to the number printed on the capacitor's body.
- Starter capacitors are rated in microfarads and voltage; if you need a replacement, the new capacitor must match both numbers exactly or your motor will not start correctly.
Safety steps before you test anything
Capacitors store electrical energy even after the power is turned off. A charged capacitor can deliver a shock that is painful or dangerous, so you must discharge it before you touch the terminals. Turn off and unplug the motor, then locate the capacitor — it is usually a cylindrical can mounted near the motor or inside the motor housing.
Take an insulated screwdriver (one with a plastic or rubber handle) and touch both terminals of the capacitor at the same time with the metal tip. You may see a small spark; that is the capacitor discharging and is normal. Do this two or three times to make sure the charge is fully gone. Now you can safely handle the capacitor and connect your multimeter.
Testing with the resistance (ohms) method
The resistance test works on any multimeter and does not require a capacitance setting. Set your multimeter dial to the resistance setting, usually marked with the Greek letter omega (Ω) or the word "ohms." Start at the highest resistance range if your meter has multiple ranges.
Touch one multimeter probe to each terminal of the capacitor. A good capacitor will show a rising resistance reading — it may start at zero and climb toward infinity (often shown as "1" on a digital display), or it may jump directly to a high number. This rising needle or climbing number means the capacitor is charging through the meter and is still good. If the meter stays at zero or shows no change, the capacitor has failed and needs replacement.
Reverse the probes and repeat the test. You should see the same rising resistance pattern. If you get different results on the second test, the capacitor is likely failing and should be replaced.
Testing with the capacitance setting (if your meter has one)
Many modern multimeters include a dedicated capacitance setting marked with a capacitor symbol or the letters "µF" (microfarads). This method is more direct than the resistance test because it measures the actual capacitance value and lets you compare it to the rating printed on the capacitor.
Set your meter to the capacitance setting and choose the range that matches or exceeds the microfarad rating on your capacitor. For example, if the capacitor is rated 50 µF, select a range of 50 µF or higher on your meter. Touch one probe to each terminal. The meter will display a number in microfarads. A good capacitor will read within about 10 percent of its rated value — so a 50 µF capacitor should read between 45 and 55 µF. If the reading is much lower or shows zero, the capacitor has failed.
Reading the capacitor label and understanding the ratings
Every capacitor has a label printed on its body that shows two key numbers: the capacitance rating (in microfarads, written as µF or MFD) and the voltage rating (in volts, written as V). For example, a label might read "50 µF 370V" or "30 µF 250V." Both numbers matter if you need to buy a replacement.
The microfarad number tells you how much electrical charge the capacitor can store. The voltage number tells you the maximum voltage the capacitor can safely handle. If you replace a capacitor with one that has a lower microfarad rating, your motor may not start. If you use one with a lower voltage rating, the capacitor may fail quickly or overheat. Always match both numbers exactly when you order a replacement.
What different test results mean
A rising resistance reading or a capacitance reading within 10 percent of the rated value means your capacitor is still working. You can reinstall it and test your motor. If the motor still does not start, the problem is elsewhere — check the motor windings, the run capacitor (if there is one), or the motor bearings.
A resistance reading that stays at zero, or a capacitance reading that is much lower than the rated value or shows zero, means the capacitor has failed internally. The dielectric material inside has broken down and the capacitor can no longer hold a charge. You will need to replace it with a new capacitor that has the same microfarad and voltage ratings.
If the resistance reading is very high or infinite from the start, the capacitor may have an open circuit — a break inside that prevents current from flowing. This also means replacement is needed. In rare cases, a capacitor may show a short circuit, where the resistance stays at zero and does not rise. This is a sign of internal damage and the capacitor must be replaced.
After you finish testing
Once you have completed your test, discharge the capacitor again using the insulated screwdriver method, even if you tested it and found it is good. This removes any charge the meter may have added during testing and leaves the capacitor safe to handle or reinstall.
If the capacitor passed the test, you can reinstall it in your motor and reconnect the power. If it failed, note the microfarad and voltage ratings from the label, then order a replacement with those exact specifications. Starter capacitors are inexpensive and widely available from electrical supply stores and online retailers.
Frequently Asked Questions
Can a capacitor test good but still be bad?
Yes, occasionally. A capacitor can pass a resistance or capacitance test but fail under the high current demand of actual motor startup. If your motor still will not start after the capacitor tests good, the problem may be a weak capacitor that is near the end of its life, or the issue may be elsewhere in the motor circuit. Consider replacing the capacitor anyway if it is more than five years old.
What if I do not have a multimeter with a capacitance setting?
The resistance (ohms) test works on any multimeter and is reliable for determining whether a capacitor is completely failed. You do not need a capacitance setting to know whether the capacitor is good or bad, though a dedicated capacitance setting gives you more precise information.
Is it safe to test a capacitor while it is still connected to the motor?
No. Always disconnect the capacitor from the motor before testing. Testing a connected capacitor can damage your multimeter or give you a false reading because current from the motor circuit can interfere with the test. Disconnect the wires, discharge the capacitor, and then test it in isolation.
Can I replace a starter capacitor with a run capacitor?
No. Starter and run capacitors are designed for different purposes and have different internal construction. A starter capacitor is built to handle high current for a short time during startup. A run capacitor is built to handle lower current continuously. Using the wrong type will damage your motor or cause it to overheat.
How often do starter capacitors fail?
Starter capacitors typically last five to ten years, depending on how often the motor runs and the temperature of the environment. Motors that start frequently or run in hot conditions wear out capacitors faster. If your motor is more than ten years old and showing startup problems, the capacitor is a likely culprit.