What does an AC capacitor do, and why does it fail?
A capacitor gives your AC's motors the jolt of stored energy they need to start and keep spinning. Most outdoor units use a single dual run capacitor that serves both the compressor and the condenser fan motor, so one bad part can take down the whole system. When it weakens, the motors cannot get going, which is why a dying capacitor usually shows up as a unit that hums or clicks but will not start the fan or compressor.
Capacitors fail from heat and age more than anything else. Sitting in a metal box in the sun, cycling on and off all summer, they slowly lose capacitance until they cannot deliver the starting kick. **Most run capacitors last about 10 to 20 years**, but high heat, an undersized part, or a motor that is itself struggling will burn one out far sooner. They are cheap and they wear out, so this is a normal maintenance item, not a sign your whole system is failing.
How do I tell if my AC capacitor is bad?
The clearest sign of a bad capacitor is a unit that hums or buzzes but will not spin up, often with the outdoor fan sitting dead still. You may also hear a click from the contactor when the system tries to start, see the fan turn slowly or only after you give the blade a push with a stick (a classic bad-capacitor symptom), or notice the breaker tripping as the motor draws too much current trying to start.
A visual check confirms it fast. **A failing capacitor often bulges, domes at the top, or leaks oily fluid**, and a swollen top is a near-certain sign it is done. The definitive test is electrical: with the capacitor safely discharged and disconnected, set a multimeter to capacitance and read each terminal pair. **A reading more than about 6 percent below the printed microfarad (uF) rating means replace it.** A capacitor printed 45/5 uF that reads 38 uF on the compressor side is bad.
Do not assume the capacitor is the only problem. If the new one fails quickly, the motor it serves is likely drawing too much and dragging the capacitor down with it, which is a job for a pro.
Can I replace an AC capacitor myself?
Yes, most homeowners can safely swap a capacitor if they respect the charge it holds and follow the steps in order. It is a simple mechanical job: two or three wires, a mounting strap, and a part that usually costs **$15 to $40**. The skill required is low. The danger is that the capacitor stays charged after you cut power, so the entire risk of this repair lives in one step, discharging it, which I cover next.
Call a pro instead if any of these are true: you are not comfortable working near 240-volt equipment, the wiring is corroded or you cannot tell which wire goes where, the capacitor keeps failing (a motor problem), or your system is under warranty that a DIY repair would void. There is no shame in it. A service call for this runs about $150 to $400, and a tech will also spot a struggling motor that would eat the new part.
If your unit is old and this is one of several recent failures, weigh repair against replacement before spending more. When you do price a new system, size it correctly with the BTU calculator rather than guessing, and an oversized unit cycles too often and wears parts out faster.
How do I safely discharge and replace the capacitor?
First, kill all power. **Turn off the AC at the thermostat, then shut off the dedicated breaker in your panel, then pull the outdoor disconnect block** (the box on the wall next to the unit). Cutting power at all three points is what keeps you safe; do not skip the disconnect just because the breaker is off.
Next, discharge the capacitor, the one step you cannot skip. Even with power off, the capacitor holds a charge that can shock you. Take an insulated-handle screwdriver and lay the metal shaft across two terminals at once to short them out, then repeat across each pair of terminals. On a dual capacitor, discharge every combination of the C, FAN, and HERM terminals. You may see a small spark. A 20,000-ohm resistor across the terminals is the safer way to bleed it down if you have one.
Now photograph the wiring before you touch it, so you can copy it exactly. Note which color wire lands on C (common), FAN, and HERM (compressor). The terminals are labeled on the top of the capacitor. Pull the wires off one at a time, loosen the metal mounting strap, and lift the old capacitor out.
Set the new capacitor in the strap, tighten it, and reconnect each wire to the matching labeled terminal from your photo. Push the connectors on firmly. Then reverse your power steps: reinsert the disconnect, flip the breaker on, and set the thermostat to cool. The unit should start within a minute. If it still only hums, cut power again and recheck your connections, then suspect the motor.
How do I pick the right replacement capacitor?
Match three things exactly: the microfarad rating, the voltage rating, and the terminal configuration. **The microfarad (uF or MFD) rating must match the old part exactly**, for example 45/5 uF on a dual capacitor (45 for the compressor, 5 for the fan). This number is printed on the side of the old capacitor and is not something to estimate; the wrong uF will damage the motors.
**The voltage rating must be equal to or higher than the original, never lower.** Most run capacitors are rated 370V or 440V, and a 440V part safely replaces a 370V one because the rating is a maximum, not a target. Going higher is fine and often more durable; going lower will fail. Also match the type: a dual run capacitor (three terminal groups: C, FAN, HERM) cannot be swapped for two single capacitors unless you rewire, so buy the same style.
Buy a name-brand run capacitor, not the cheapest listing. Brands like Titan, TradePro, and Genteq are widely used and reliable. A quality dual run capacitor in the common 45/5 or 35/5 uF sizes runs about $15 to $30. Bring the old part's printed specs to the store or have them on screen when you order, because close is not good enough here.
What does it cost to replace an AC capacitor?
Doing it yourself costs only the part, **about $15 to $40** for a standard dual run capacitor. That, plus a multimeter and an insulated screwdriver you may already own, is the whole bill. It is one of the best value DIY HVAC repairs precisely because the part is cheap and the labor markup on a service call is large.
Hiring a pro typically runs **$150 to $400 installed**, with the part marked up and a diagnostic and labor fee on top. That premium buys you a trained eye: a good tech will check the motor's amp draw to confirm it is not the reason the old capacitor died, which prevents you from burning through a second part. If your system is also low on refrigerant or showing other age, get the whole unit assessed before pouring money into piecemeal repairs.
Frequently asked questions
Can I replace my AC capacitor myself?
Yes, most homeowners can. It is a simple swap of two or three wires and a cheap part, and the work itself is easy. The one real hazard is that the capacitor holds an electrical charge even after you cut power, so you must turn off the thermostat, breaker, and outdoor disconnect, then short across the terminals with an insulated screwdriver to discharge it before touching any wires. Call a pro if you are not comfortable near 240-volt equipment or if the capacitor keeps failing.
How do I tell if my AC capacitor is bad?
The classic signs are a unit that hums or buzzes but will not start, a condenser fan that sits still or only spins after you nudge it, and a breaker that trips on startup. Visually, a bad capacitor often bulges or domes at the top or leaks oily fluid. The sure test is to discharge it, disconnect it, and read it with a multimeter on the capacitance setting: a value more than about 6 percent below the printed microfarad rating means it is bad.
What is the average cost to replace a capacitor on an AC unit?
Doing it yourself costs about $15 to $40 for the part alone. Hiring an HVAC technician typically runs $150 to $400 installed, which includes the marked-up part plus a diagnostic and labor fee. The capacitor is one of the cheapest AC components, so most of a professional bill is labor, which is why this is a popular DIY repair for anyone comfortable discharging the part safely.
What is the $5000 rule for AC?
The $5000 rule is a quick way to decide whether to repair or replace a unit: multiply the age of the system in years by the repair quote in dollars, and if the result is over $5,000, replace the unit instead of fixing it. A 10-year-old system facing a $600 repair scores 6,000, so replacement is the smarter spend. A capacitor swap almost never crosses that line, but a failing compressor on an old unit usually does. If you are leaning toward replacement, size the new system with the BTU calculator.
Do I have to discharge an AC capacitor before replacing it?
Yes, always. A run capacitor stores a charge that can shock you even after the power is off, so discharging is the single most important safety step. After cutting power at the thermostat, breaker, and outdoor disconnect, lay the metal shaft of an insulated-handle screwdriver across two terminals at a time to short them out, repeating across every pair. A 20,000-ohm resistor is a gentler way to bleed the charge if you have one.