A handful of components account for most HVAC repair calls - the compressor, the capacitor, the coil and the blower motor. This section covers what each one does, what its failure looks like, and what determines whether repairing it or replacing the whole system is the better call.
A repair quote is easier to evaluate once you know what the failed part actually does and roughly how often it fails. This section is organized by component, not by symptom - Common Problems starts from what you're noticing instead.
The components behind most repair calls
Capacitor. Gives the compressor and fan motors the jolt of power they need to start. One of the most common AC failures and, on its own, one of the least expensive parts to replace - a system that will not start at all is often just this.
Compressor. The core of the outdoor unit; it is what actually moves refrigerant through the system. The single most expensive component to replace, and its failure is often what pushes the repair-vs-replace decision toward replacing the whole outdoor unit rather than just the part.
Coil (evaporator or condenser). Where heat transfer actually happens. A leaking coil causes slow refrigerant loss and gradually worse cooling performance before it fails outright. Repair is possible; on an older system it is also a common point where replacing the whole unit gets seriously discussed.
Blower motor. Moves air through the ductwork. A failing blower motor often shows up as weak airflow or unusual noise before it stops entirely.
Thermostat. The least expensive component on this list to replace, and worth ruling out first - a system that "won't turn on" is sometimes a thermostat problem having nothing to do with the furnace or AC unit itself.
Repair or replace the whole unit
The rule of thumb technicians use, worth knowing even though it is only a starting point: weigh the repair cost against the system's age and remaining expected life. A major-component failure (compressor, coil) on a system already near the end of its typical lifespan tends to tip toward replacement; the same failure on a newer system tends to tip toward repair. Get both numbers before deciding either way.
Common questions
Is it worth repairing an old system at all?
Depends on the age, the specific failure, and what a full replacement would cost instead. A cheap fix (capacitor, thermostat) on an old system is usually still worth doing; a compressor or coil failure on an old system is the point where getting a replacement quote alongside the repair quote is worth the extra call.
Why did my capacitor fail?
Capacitors have a service life and typically fail from age, heat stress, or voltage fluctuations - it is a genuinely common failure rather than a sign something else is wrong, though a technician should confirm nothing else caused it.
Can I replace a capacitor myself?
It stores an electrical charge even when the system is off, which makes it a real shock hazard without the right tools and discharge procedure. See DIY vs Professional for which HVAC tasks are and are not reasonable to do yourself.