Quick answer
A bad AC capacitor shows up as an outdoor unit that hums without starting, a fan that needs a push, slow or hard starts, clicking, or warm air on hot afternoons only. The top of the can may be bulged or leaking. A technician confirms it with a meter: a reading more than about 6% below the rated microfarads means replace it, typically $150–$350.
You know your AC capacitor is bad when the outdoor unit has power and a cooling signal but a motor will not start, or struggles to. The capacitor is the silver can in the electrical compartment that gives the compressor and fan motor the electrical push they need to start and keep running. Replacing it is the most common AC repair and one of the least expensive. It is also the one part of the unit a homeowner should not handle.
Symptoms of a failing AC capacitor
- Humming outdoor unit: the contactor has pulled in and the compressor or fan is trying to start but cannot.
- A fan that needs a push: the blades sit still or twitch, and spin up if nudged. That is the fan side of the capacitor, and the right response is to switch the unit off, not to keep nudging it.
- Slow or hard starts: the unit shudders, the lights dim longer than usual, or it takes several seconds to come up to speed.
- Clicking: the compressor stalls, its thermal overload opens, and it retries every few minutes.
- Warm air on hot afternoons only: a weak capacitor can start the compressor on a mild morning but not against the higher pressures of a 32 °C afternoon.
- Longer run times and a higher hydro bill, because a motor on a weak capacitor draws more current.
Several of these overlap with other faults: a fan that is not spinning can also be a seized motor, and a buzz with no start can be a contactor.
Visual signs and the meter test against the rated microfarads
A healthy capacitor has a flat top. One that has failed often has a domed or bulged top, a split seam, oily residue or rust around the terminals. The bulge is a built-in pressure release doing its job, and any bulge means replace it, whatever a meter says. A flat top proves nothing, though: many weak capacitors look perfect.
Look, but do not touch. The capacitor is only visible with the access panel off, and behind that panel are 240-volt terminals that are live unless the disconnect is pulled, and a charged capacitor even when it is.
The definitive test is a capacitance reading compared with the rating printed on the label. Most residential condensers use a dual-run capacitor: one can with three terminals marked C, HERM and FAN. A label reading 45/5 µF means 45 microfarads for the compressor and 5 for the fan; 35/5 and 45/5 are the sizes seen most often on house-sized units.
The label also gives a tolerance, commonly plus or minus 6%. A 45 µF section that measures below about 42.3 is out of tolerance and gets replaced; so does a 5 µF fan section reading under 4.7. The replacement must match the microfarad values exactly, and its voltage rating, 370 or 440 volts, must be equal or higher. A technician can also test it under load with the unit running, which catches capacitors that read fine cold and fade when hot.
Why this is not a DIY test: capacitors hold a charge
A capacitor stores energy, and it can hold a dangerous charge after the power is off. Pulling the disconnect does not empty it. A technician discharges it through a resistor and confirms zero volts before touching the terminals; the screwdriver-across-the-terminals method can weld the tool, damage the capacitor and injure you. Add the 240-volt line terminals a few centimetres away, and this is a job to watch, not do.
What happens if a failed capacitor is left in place
A weak capacitor does not fail alone. A motor running on low capacitance draws higher current and runs hot, and heat destroys winding insulation. Left for a season, the capacitor takes out the fan motor or the compressor it serves.
- Capacitor replacement: $150–$350.
- Condenser fan motor: $300–$700.
- Compressor: the most expensive part in the unit and, on an older system, a repair-or-replace decision. See what an AC compressor costs.
- Diagnostic visit: $90–$150, which comes off the repair bill.
The rule is simple: if the unit hums and will not start, turn it off at the thermostat and leave it off until it is tested. Every retry heats the windings.
Why capacitors fail in the first heat wave
Capacitors are wear parts, and heat is what ages them, which is why so many quit during the first hot week of the year. A capacitor that tests at the edge of its tolerance during a spring tune-up is worth replacing then, not in July. Testing it against the label is a standard line on a tune-up.
A capacitor is tested and changed on a normal AC repair visit, with the price given in writing before the part goes in. ZK Mechanical covers Burlington, Oakville, Milton, Hamilton, Mississauga and Brampton: call (647) 801-1252 or book online. The part itself is explained in the glossary entry for the AC capacitor, and neighbouring symptoms are in the cooling answers.