In short
An AC capacitor is the can-shaped part in the outdoor unit that gives the compressor and fan motors the electrical push to start and stay running. It is rated in microfarads, and a reading outside the tolerance on its label, commonly ±6%, means it is due. Replacement runs $150–$350 in the GTA. It holds a charge with the power off, so leave it to a technician.
What is an AC capacitor? It is the silver can, about the size of a pop can, wired into the electrical compartment of your outdoor unit. The compressor and the condenser fan are single-phase motors that cannot start or run properly on household power alone, and the capacitor supplies the shifted current that gets them turning and keeps them on speed. When it weakens the motors struggle; when it fails they sit and hum.
What an AC capacitor does: run, start and hard-start kits
Most residential condensers use one dual run capacitor that serves both motors. It has three terminals marked C (common), HERM (the compressor) and FAN, and it stays in the circuit the whole time the unit runs. Three terms show up on invoices, and each is a different part.
- Run capacitor: stays in the circuit continuously. The dual version carries two ratings in one can, for example 45/5 µF, the larger for the compressor and the smaller for the fan.
- Start capacitor: a much larger value in a black plastic case, in the circuit for a fraction of a second at start-up and then dropped out by a relay.
- Hard-start kit: a start capacitor and relay sold as one add-on, fitted to help an aging compressor get moving or one fed by a long, weak electrical run.
A hard-start kit on a healthy system is not a repair. If one is proposed, ask what the compressor's start-up amp draw was, because a kit can mask a compressor that is on its way out. The AC compressor entry covers that decision.
How an AC capacitor fails and the numbers on its label
Heat kills capacitors. Across Burlington, Hamilton and the west GTA the calls come in a wave on the first 30 °C days and again in every heat wave, because the can sits in a metal box in direct sun while the unit runs long cycles. The label gives three things: the microfarad rating (written µF, uF or MFD), a tolerance that is commonly ±6%, and a voltage of 370 or 440 VAC. A technician measures the actual microfarads with a meter, and a 45 µF section that reads 40 is out of tolerance and gets replaced even if the unit still starts.
- The outdoor unit hums but the fan blade sits still.
- The top of the can is bulged or domed, or there is oily residue on it.
- The compressor tries to start, clicks off, and tries again a few minutes later.
- The lights dim hard each time the AC kicks in, or the breaker trips at start-up.
- The indoor blower runs and the air from the vents is room temperature.
Turn the system off at the thermostat if you see any of these. A compressor that keeps trying to start against a dead capacitor overheats, and that turns a small repair into a large one. Do not spin the fan blade with a stick to get it going; it proves nothing and puts your hand beside live parts. Our answer on why an AC is not turning on lists the checks that are safe from outside the cabinet.
What AC capacitor replacement costs in Ontario, and why it is not a DIY part
- Diagnostic fee: $90–$150, and it comes off the repair price.
- Capacitor replacement, part and labour: $150–$350.
What you are paying for is a correct match and a tested system. The replacement must carry the same microfarad values and a voltage rating equal to or higher than the original: a 440 V can may replace a 370 V one, never the reverse. The full breakdown is in how much an AC capacitor costs.
A capacitor holds its charge after the disconnect is pulled and the breaker is off. It can deliver a serious shock until it is discharged with the right tool, and the compartment around it has line-voltage terminals that stay live on many units. That is why we do not coach this repair over the phone.
What to do next if you suspect the capacitor
Shut the AC off at the thermostat, leave the fan setting on auto, and book AC repair. A capacitor is normally a one-visit job: we test the old part with a meter in front of you, then check the contactor and the compressor's amp draw before closing the panel, so the new capacitor is not covering for another fault. Call (647) 801-1252 or send the details through the contact page.
Questions people ask about AC capacitor
Can I replace an AC capacitor myself?
We advise against it. The capacitor stores a charge that can shock you with the breaker off, and the same compartment holds 240-volt terminals that stay live on many units until the outdoor disconnect is pulled. A wrong microfarad value also damages the compressor slowly. A licensed technician does the swap, confirms the readings and checks why the old one failed.
How long does an AC capacitor last?
There is no fixed number. Some last the life of the unit and some fail in a few seasons; heat, long run times, power surges and a failing motor drawing high amps all shorten the life. That is why a tune-up includes a microfarad reading: a capacitor drifting toward the edge of its tolerance can be changed in spring instead of on the hottest Saturday in July.
Will a bad capacitor damage the compressor?
Yes, if the unit keeps running on it. A weak run capacitor makes the compressor draw higher current and run hotter, and a dead one leaves it stalled and humming until its internal overload trips. Repeated stalled starts cook the windings. Switching the system off at the thermostat as soon as you notice the hum is the cheapest thing you can do.