A capacitor is the cheapest thing that stops an AC.

A run capacitor is a metal cylinder, usually about the size of a soda can, sitting inside the electrical compartment of the outdoor unit. Its job is to give the compressor and the fan motor the shove they need to start, and then to keep them running efficiently.

It is also the part most likely to fail on the hottest day of the year, because heat is what wears it out. In a Phoenix summer it is the single most common thing that stops a working system.

What it looks like when one is going

The compressor hums but does not start. You hear a low buzz from the outdoor unit for a few seconds, then it cuts out. That is a motor trying to start with nothing to start it.

The fan on top sits still while the unit runs. The compressor may still be going. Without the fan, the unit cannot reject heat, pressures climb, and it will eventually trip on its own high-pressure switch.

The system trips the breaker in the afternoon. A motor that cannot start pulls locked rotor current, which is several times its normal draw. The breaker is doing exactly what it should.

The unit starts fine in the morning and quits by three. Marginal capacitors fail under heat first. A system that works at 7am and gives up in the afternoon is a classic.

The top of the can is domed or leaking. A healthy capacitor has a flat top. Swollen, split, or weeping oil means it is done. You will only see this if the panel is off, which is the next section.

Do not open the panel

This is the part worth being blunt about. A capacitor holds its charge after the disconnect is pulled. It does not care that the power is off. Discharging one across your hand is a genuine injury, not a tingle.

There is a trick where a stuck fan is nudged with a stick to see whether it will run once started. It is a real diagnostic and it tells you a lot. It is also a spinning blade in a live electrical cabinet, and it is not worth doing at home for the sake of a part this cheap.

The other half of the problem is buying the right one. Capacitors are rated in microfarads and in volts, and both matter. An undersized replacement will run a motor hot and take the motor with it, which turns a small job into a much larger one.

What it costs

The price depends on which motor the capacitor serves and where the unit sits, and that is already built into the number. Nothing gets added for a roof. The whole flat-rate price list is published, and there is a longer write-up on what a repair costs here.

Where the fault is not obviously a capacitor, it starts as a diagnostic, and the repair price comes off the published list once the fault is known. If the symptom is a house that will not cool rather than a unit that will not start, begin with the no-cooling checklist. You hear the number and agree to it before anything gets touched.

Worth knowing before you book

A capacitor replacement is a short job. If someone quotes you four figures to deal with one, that is worth a second set of eyes before you sign anything. A second opinion on a written quote is on the price list, and it is cheaper than being wrong.