AC compressor repair usually turns out to be a repair to something next to the compressor, because a failed run capacitor, a burnt contactor or a tripped thermal overload produces the same symptoms as a dead compressor and costs a fraction as much to fix. When the compressor itself has failed (shorted or grounded windings, or a seized motor), it can't be repaired; it gets replaced, and that's one of the most expensive repairs on a central AC. This guide covers how to tell the difference, what drives the replacement cost, and when replacing the whole system is the better call.
What Is an AC Compressor?
An AC compressor is the pump that moves refrigerant through your air conditioner. On a central AC or heat pump it sits inside the outdoor unit: the dark, dome-shaped canister in the middle of the cabinet, surrounded by the condenser coil. It squeezes low-pressure refrigerant vapor returning from the indoor coil into a hot, high-pressure gas, which the outdoor coil and fan cool down. Residential compressors are hermetically sealed in a welded steel shell, which is why internal failures mean replacement. In a heat pump, the same compressor serves both heating and cooling, so HVAC compressor repair on a heat pump follows the same diagnosis, repair and replacement rules covered on this page.
Three designs show up in homes:
- Reciprocating. Pistons in cylinders, like a small engine. Common in older central systems, with more moving parts and more noise.
- Scroll. One spiral orbits against a fixed one. Most current central ACs and heat pumps use scroll compressors, which have fewer moving parts and run quieter.
- Rotary. A roller turning inside a cylinder. Compact, so it's the usual choice in mini splits, window units and portable ACs.
Compressors also differ in how they deliver capacity. A single-stage compressor is either off or at full output. A two-stage compressor adds a reduced low stage for mild days, giving longer, quieter cycles and better humidity control. A variable-speed (inverter) compressor uses an electronic drive to vary motor speed with the load; it's standard in mini splits and higher-tier central systems. On inverter equipment, a failed drive board can mimic a dead compressor.
Signs of a Failing Compressor and the Cheaper Parts That Mimic It
Most classic "bad compressor" symptoms have a cheaper explanation.
| Symptom | Cheaper part that often causes it | When it points to the compressor |
|---|---|---|
| Outdoor unit hums, fan may run, compressor won't start | Failed or weak run capacitor | Capacitor tests good but the compressor still won't start and draws locked-rotor amps |
| Nothing happens outdoors when cooling is called | Burnt or pitted contactor, no 24-volt signal, tripped breaker | Contactor closes and voltage reaches the compressor, but it doesn't run |
| Compressor starts, runs a few minutes, shuts off, restarts later | Thermal overload tripping from a dirty condenser coil, dead fan motor or low charge | Overload keeps tripping after the coil, fan and charge are corrected |
| Struggles to start, lights dim, sometimes trips the breaker | Weak capacitor, loose connections | Starts only with a hard-start kit, or windings test close to shorted |
| Breaker trips the instant the compressor tries to start | Damaged wiring at the unit | A winding reads continuity to ground (a grounded compressor) |
| Warm air, outdoor unit runs normally | Low refrigerant from a leak, dirty filter, frozen coil | Pressures stay close together and amp draw is low: the compressor isn't pumping |
| Grinding, rattling or knocking | Loose fan blade, worn fan motor bearings, debris | Noise comes from inside the compressor shell |
Run capacitor and contactor. A weak or bulging run capacitor is the most common cause of a humming, non-starting outdoor unit. A contactor with burnt points, or an insect wedged between them, keeps power from ever reaching the compressor.
Hard-start kit. An add-on start capacitor and relay that gives an aging compressor an extra kick at startup. It helps a healthy compressor that struggles to start, not one with shorted windings or locked internals.
Thermal overload trip. The compressor has an internal overload that opens when it gets too hot. A tripped overload looks exactly like a dead compressor and can take an hour or more to reset. The cause is usually a dirty outdoor coil, a dead condenser fan, a low charge or a weak capacitor. Condemning a hot, tripped compressor is a classic misdiagnosis.
For other reasons the system blows warm air, see our AC not cooling troubleshooting guide.
What You Can Check, and What Not to Touch
You can safely check that the thermostat is set to Cool below room temperature, reset a tripped breaker once, confirm the outdoor disconnect is on, replace a dirty filter and clear debris from around the outdoor unit. If the unit hums without starting, or the breaker trips again, switch cooling off and leave it off. Every failed start attempt heats the compressor motor.
Don't go further than that:
- Capacitors hold a charge even with the power off and can deliver a serious shock. Leave them alone.
- Don't open the refrigerant circuit or add refrigerant. Handling refrigerant legally requires EPA Section 608 certification.
- Don't keep resetting the breaker or "help" the fan start. Both can burn out a compressor that only needed a capacitor.
How a Technician Diagnoses the Compressor
A proper diagnosis involves several measurements. Ask to see the readings.
- Capacitor test. Discharge it and measure capacitance against the rating on the label. Most compressor callbacks start and end here.
- Voltage at the compressor terminals. Confirms the contactor is closing and power is getting there.
- Amp draw. Measured with a clamp meter and compared with the rated load amps (RLA) and locked-rotor amps (LRA) on the nameplate. Current near LRA that doesn't drop means the motor is locked or not starting.
- Winding resistance. With power off, the tech measures between the common, start and run terminals. Start-to-run should equal common-to-start plus common-to-run. An open reading can mean a tripped internal overload (let it cool and retest) or a broken winding. A near-zero reading means shorted windings.
- Ground test. A winding with continuity to the shell is a grounded compressor. It's finished, and dangerous to keep energizing.
- Pressures and temperatures. If it runs, suction and discharge pressures, superheat and subcooling show whether it's pumping or whether the problem is a low charge or a restriction.
A compressor quote without this testing, especially without a capacitor check, isn't a diagnosis.
What Compressor Replacement Involves
The tech recovers the refrigerant, unbrazes the old compressor, brazes in the new one and replaces the liquid-line filter drier. After a burnout, they may test the oil for acid and add a suction-line drier. The system is then pressure-tested with nitrogen, evacuated to a deep vacuum and recharged by weight, usually with a new capacitor and contactor. Expect most of a day of work, plus time to source the part.
AC Compressor Replacement Cost: What Drives the Price
A capacitor, contactor or hard-start kit sits at the low end of AC repair costs. Compressor replacement sits at the top tier, alongside coil replacement. Where a given job lands within that tier depends on:
- Warranty: parts versus labor. Many manufacturers cover compressors for 5 or 10 years on parts, and the longer term often requires registering the equipment. A covered part comes off the bill, but labor, refrigerant and the drier usually don't, so a warranty job is still a major repair. Check the manufacturer's current warranty terms for your model and registration status.
- Refrigerant type and amount. The charge is recovered and replaced. R-22 is the most expensive tier because US production ended on January 1, 2020, leaving only reclaimed and stockpiled supply. R-410A is still used to service existing systems, though its supply is being phased down as new equipment moves to R-454B and R-32 (see the R-410A phase out). Bigger systems and long line sets hold more refrigerant.
- Compressor type and tonnage. Larger tonnage costs more. Two-stage and variable-speed compressors cost more than single-stage, and inverter models may need a matching drive board.
- Line set and driers. A new filter drier is standard. An acid-producing burnout adds a suction-line drier, a flush or a new line set.
- Access and timing. Rooftop and tight-access units take longer, and after-hours calls cost more.
- System age. On an older unit the original compressor may be discontinued and need a substitute, and the money goes into a system whose coil, fan motor and controls are just as old.
For typical ranges, see AC repair cost by type of repair.
Should You Repair or Replace the Compressor?
First rule out the capacitor, contactor and a tripped overload with meter readings. Then there are three calls: repair the part, replace the compressor, or replace the system. Central ACs typically last about 15 to 20 years, so age carries a lot of weight.
| System age | Warranty status | Refrigerant | Other failing parts | Repair cost vs half the price of a new system | The call |
|---|---|---|---|---|---|
| Any | Any | Any | Compressor tests good; the capacitor, contactor or overload is the real fault | Well under half | Repair the part |
| Under 10 years | Active, or expired with the rest of the system sound | R-410A or newer | None | Under half | Replace the compressor (get a system quote too if out of warranty) |
| 10 to 15 years | Active parts warranty | R-410A | None; coil and fan motor sound | Under half | Replace the compressor |
| 10 to 15 years | Expired | R-410A | Leaking coil, worn fan motor or repeated repairs | Near or over half | Replace the system |
| Any | Any | R-22 | Any | Usually over half | Replace the system |
| Over 15 years | Any | Any | Any | Any | Replace the system |
The 50 percent rule is a starting point: if the compressor job costs more than about half the price of a comparable new system, replacement usually wins. Compare your quote with central AC installation cost for your size of home.
Also consider matching: a new outdoor unit generally needs a matched indoor coil to reach its rated efficiency, so a full system often costs less per year of life than a new outdoor unit alone.
How to Vet a Contractor for Compressor Work
Questions to ask:
- What readings show the compressor failed? Did you rule out the capacitor and a tripped overload?
- Is the compressor still under the manufacturer's warranty, and will you file the claim?
- Will you replace the filter drier, pull a deep vacuum and weigh in the charge?
- What warranty do you give on your labor?
- Can I see quotes for both the compressor and a full system replacement?
Red flags:
- Condemning the compressor without meter readings, or while it's still hot from tripping.
- Pushing a full replacement before checking warranty status.
- Topping off refrigerant on a system that keeps losing charge.
- Pressure to sign the same day.
For other AC problems, see AC repair by symptom. To protect a new compressor, what a professional AC tune-up includes covers the checks that catch weak capacitors and dirty coils early.
Frequently Asked Questions
Can an AC compressor be repaired, or does it need to be replaced?
The compressor itself is a sealed unit, so internal failures such as shorted windings or a seized motor mean replacing it. Most "compressor repairs" are actually repairs to the parts around it: the run capacitor, contactor, a hard-start kit or wiring. Proper electrical testing tells you which situation you're in.
How do I know if my AC compressor is bad or just the capacitor?
You can't tell safely from outside. Both cause a humming outdoor unit that won't start and warm air indoors. A technician discharges and measures the capacitor against its rating, then measures compressor winding resistance and amp draw. A bad capacitor is far more common and far cheaper.
Is it worth replacing the compressor on a 10-year-old AC?
It can be if the compressor is still under its parts warranty, the system uses a current refrigerant and the rest of the unit is in good shape. Without warranty coverage, or on an R-22 system, compare the repair against replacing the whole system using the 50 percent rule.
How long does an AC compressor last?
Commonly 10 to 15 years, and in a well-maintained system it can last as long as the rest of the equipment. Hot climates, dirty condenser coils, low refrigerant charge and repeated hard starts all shorten its life.
Can I run my AC with a bad compressor?
No. A compressor that is tripping on overload or drawing locked-rotor current won't cool, and repeated start attempts overheat the motor and can contaminate the refrigerant system. Turn cooling off at the thermostat until it has been tested.