How do you check if an AC capacitor is bad?
You check a capacitor in three escalating steps: symptoms, a visual inspection, and a multimeter reading. The symptoms point the finger first. A unit that hums or clicks but will not start, a condenser fan that sits still or only spins after you nudge it with a stick, and a breaker that trips when the AC tries to start are the classic signs of a weak capacitor. If your outdoor fan is dead, our guide to why the AC fan is not spinning walks through how a capacitor failure compares to a motor or contactor failure.
The visual check comes next, after you have cut the power. A bad capacitor often bulges or domes at the top, leaks oily fluid, or shows rust and scorching around the terminals. A swollen top is close to a guaranteed verdict; capacitors are built flat, and internal failure is what pushes the lid up. If yours looks like a soda can left in the freezer, you can skip the meter and go straight to replacing it.
Plenty of failed capacitors look perfectly normal, though. Capacitance fades electrically long before anything shows on the outside, which is why the multimeter test below is the only way to clear or convict a normal-looking part.
Can I test a capacitor with a normal multimeter?
Only if it has a capacitance mode. To get an actual microfarad reading you need a meter with a capacitance setting, marked uF, a capacitor symbol (two short parallel lines), or CAP on the dial. Many basic multimeters in the $10 to $15 range only do voltage, resistance, and continuity, and those cannot measure capacitance. Mid-range digital meters with a capacitance mode start around $25 to $40, and if you plan to do any of your own AC troubleshooting it is worth owning one.
If your meter only has an ohms setting, you can still run a rough pass/fail check using the resistance method described further down. It will catch a dead-shorted or fully open capacitor, but it cannot catch the most common failure, a capacitor that still works but has drifted too far below its rating. A capacitor that is 20 percent weak will pass an ohms check and still leave your compressor struggling, so treat the resistance method as a screening test, not a verdict.
How do I safely discharge the capacitor before testing?
Cut the power at three points before you touch anything: turn the AC off at the thermostat, switch off its breaker in the panel, and pull the disconnect block from the box on the wall next to the outdoor unit. All three, every time. The disconnect is the one people skip, and it is the one that protects you if someone flips the breaker back on while your hands are in the unit.
Then discharge the capacitor, because it holds a charge even with all power removed. Lay the metal shaft of an insulated-handle screwdriver across two terminals at once to short them, and repeat across every pair. On a dual run capacitor, that means every combination of the C, FAN, and HERM terminals. You might see a small spark; that is the stored charge leaving. A 20,000-ohm resistor held across the terminals does the same job more gently if you have one. Only after discharging do you pull the wires off, and photograph the wiring first so everything goes back where it was.
If any part of this step makes you hesitate, stop and book a technician. A diagnostic visit runs about $75 to $200, and a tech will test the capacitor under load, which is a better test than a bench reading anyway. There is no version of this job worth a shock.
How do I test the capacitor with a multimeter?
With the capacitor discharged and its wires off, set the meter to capacitance and touch one probe to each terminal of the pair you are testing. Polarity does not matter on a run capacitor. Give the meter a few seconds to settle and read the microfarads. On a dual capacitor, test C to FAN for the fan side and C to HERM for the compressor side; each side has its own printed rating, written like 45/5 uF, where 45 is the compressor side and 5 is the fan side.
Now compare the reading to the printed rating, which is on the label along with a tolerance, usually plus or minus 6 percent. A reading more than about 6 percent below the printed microfarad value means the capacitor is bad. A 45/5 that reads 41 uF on the HERM side is done; a 5 uF fan side reading 4.9 is fine. Readings slightly above the rating are normal and not a problem. If the meter reads near zero or shows OL and will not settle, the capacitor is shorted or open and the verdict is the same: replace it.
One borderline case is worth knowing. A capacitor that reads just inside tolerance at the start of summer will be outside it by August, because capacitance drops further as the part heats up. If you are chasing a unit that starts fine in the morning and quits on hot afternoons, a barely-passing capacitor is a prime suspect, and at $15 to $40 for the part, replacing a borderline one is cheap insurance.
Should a capacitor have continuity?
No, a healthy capacitor should not show steady continuity. A capacitor is two conductors separated by an insulator, so a constant beep on the continuity setting means the insulation has failed and the capacitor is shorted. A brief beep or a momentary low reading that climbs is normal; that is your meter charging the capacitor through its test current, not a fault.
This behavior is also the basis of the old ohmmeter screening test. Set the meter to a high resistance range and put the probes on the terminals: a good capacitor shows low resistance that climbs steadily toward infinity as it charges. A reading stuck near zero means shorted; a reading stuck at OL that never moves means open. Swap the probes to repeat the test, since reversing them discharges and recharges the part. It is a crude check, but on a meter without capacitance mode it will catch the outright corpses.
What should I do if the capacitor tests bad?
Replace it, and match the specs exactly. The microfarad rating must match the old part exactly, and the voltage rating must be equal or higher, never lower (a 440V part safely replaces a 370V part). The full job, including picking the right part and wiring it in, is covered in our guide to changing an AC capacitor. Done yourself, the fix costs the price of the part; done by a pro, expect $150 to $400 installed.
If the capacitor tests fine, the meter just saved you from replacing a good part, and the search moves to the contactor, the fan motor, or the compressor. Our AC not working guide walks the full diagnostic order. And if the unit is old and this is the latest in a string of repairs, weigh the repair bill against replacement before spending more; when you price a new system, size it with the BTU calculator instead of assuming the old unit was sized right.
Frequently asked questions
How do you check if a capacitor is bad?
Start with the symptoms: an AC that hums but will not start, a fan that needs a push to spin, or a breaker that trips at startup. Then cut all power, discharge the capacitor, and look for a bulging top or oily leak, either of which means it is bad. The definitive check is a multimeter on the capacitance setting: a reading more than about 6 percent below the microfarad rating printed on the label means replace it.
Can I test a capacitor with a normal multimeter?
Only if it has a capacitance (uF) setting, which many basic meters lack. Without that mode you can run a rough resistance test, where a good capacitor shows a reading that climbs from low toward infinity as it charges, but that only catches shorted or open capacitors. It misses the most common failure, a capacitor that has drifted below its rated microfarads, so a meter with a capacitance mode is the right tool.
Should a capacitor have continuity?
No. A constant beep on the continuity setting means the capacitor is internally shorted and needs replacing. A brief beep or a momentary low reading that quickly climbs is normal, because the meter's test current charges the capacitor for an instant. Steady continuity is the failure, not the passing grade.
How does a multimeter test capacitance?
The meter applies a small known current to the capacitor, measures how fast the voltage rises across it, and calculates capacitance from that charge rate. That is why the reading takes a few seconds to settle and why the capacitor must be discharged and disconnected first; a stored charge or a parallel circuit path will throw the measurement off or damage the meter.
Can I test an AC capacitor without removing it?
You should at least disconnect its wires, even if you leave it strapped in the unit. Testing with the wires attached puts the motors in parallel with the capacitor and gives a false reading. Cut power at the thermostat, breaker, and outdoor disconnect, discharge the capacitor across every terminal pair, photograph the wiring, then pull the wires off and test terminal to terminal. Technicians can test under load with a clamp meter, but that is a pro technique on live equipment, not a DIY method.