The compressor is the heart of the system, and it is often blamed unfairly
The compressor is the pump that circulates refrigerant between the indoor coil and the outdoor unit, raising its pressure and temperature so heat can be released outdoors. It is also the most expensive component in an air conditioner or heat pump. When cooling stops and the outdoor unit hums, clicks or trips the breaker, the compressor is the first suspect. It should not automatically be the verdict.
A compressor depends on several smaller parts to start and run. The run capacitor gives the motor the phase shift it needs to turn. The contactor switches line voltage on when the thermostat calls. Pressure switches and thermal overloads shut it down to protect it. If any of these fails, the compressor sits idle or struggles, and to someone standing next to the unit the symptoms look identical to a motor failure. Replacing a capacitor or contactor is a routine repair. Replacing a compressor is a major one.
So we test before we conclude. Winding resistance between common, start and run terminals tells us whether the motor windings are intact. A megohmmeter or ground test tells us whether a winding has shorted to the shell. Amp draw at startup and during operation shows whether the motor is locked, laboring or healthy. Only when those tests point to the compressor itself do we talk about replacing it.
Warning signs of compressor trouble
Some of these point to the compressor itself, others to the parts around it. All of them justify a diagnostic visit.
- Outdoor unit hums but never startsOften a failed capacitor, sometimes a locked compressor. A test separates the two quickly.
- Breaker trips a few seconds after startupCan indicate a shorted or grounded winding, but also a weak capacitor causing excessive starting current.
- Loud clanking, knocking or grinding outsideInternal mechanical wear or a broken mount inside the shell. Shut the system off and schedule an inspection.
- System runs but cooling is weakA compressor with worn valves may run without building proper pressure, which shows up clearly on gauges.
- Compressor starts, stops, starts againShort cycling from an overheating compressor, a faulty pressure switch or an oversized system all look similar from indoors.
How we diagnose a compressor problem
Each step either clears a component or points to it, so the conclusion is based on evidence.
Visual and power check
We confirm supply voltage at the disconnect, look for burned wiring and inspect the contactor points for pitting.
Capacitor test
The run capacitor is measured against its rated capacitance, and dual capacitors are checked on both the compressor and fan sides.
Winding resistance
Resistance between compressor terminals is compared with expected values to detect open or shorted windings.
Ground test
A high-voltage insulation test shows whether any winding has broken down to the compressor shell, which is a condition that cannot be repaired.
Operating readings
If the compressor runs, amp draw, suction and discharge pressures and temperatures are measured to assess its pumping ability.
Told you need a new compressor?
It is worth a second set of readings before a major expense. Call and a technician will retest the start components and windings.
Repair, hard-start kit or full replacement
When the fault is a capacitor, contactor or wiring issue, the repair is straightforward and the compressor goes back to normal service. When a compressor is mechanically sound but struggles to start, particularly an older unit with high starting current, a hard-start kit that adds a start capacitor and relay can extend its life. That is a legitimate repair in the right circumstances, not a cure for a compressor with damaged windings.
A compressor with a grounded winding, a locked rotor that will not turn or worn internal valves needs replacement. The decision then becomes whether to replace the compressor alone or the whole outdoor unit. Age of the system, remaining warranty coverage, refrigerant type and the condition of the coils all feed that decision. On a young system with a parts warranty, compressor replacement often makes sense. On an older R-22 system, it rarely does, and the options on our Ac Installation page become the better path.
What a compressor replacement involves when it is warranted
Replacing a compressor is a substantial repair. The refrigerant is recovered, the old compressor is cut out, and a new one is brazed in along with a new filter drier. If the old compressor burned out electrically, the system may contain acid, and cleanup with suction line filters and an acid test becomes part of the job. The system is then pressure tested, evacuated to a deep vacuum and recharged, and the run capacitor and contactor are typically replaced at the same time so a new compressor is not paired with worn start components.
Many compressors carry a longer manufacturer warranty than the rest of the unit, but the warranty usually covers the part only. Labor, refrigerant and the supporting components are separate, which is why the total cost can approach that of a new condenser on some systems.
Protecting a compressor so it lasts
Compressors fail most often because of conditions around them: low refrigerant charge that lets them overheat, dirty condenser coils that raise head pressure, electrical stress from weak capacitors, and liquid refrigerant returning from a frozen or flooded indoor coil. Regular maintenance addresses all four. An annual Ac Tune Up that cleans the condenser coil, tests the capacitor and verifies charge is the least expensive insurance a compressor can have.
Compressor repair questions
How can I tell if my AC compressor is bad or it is just the capacitor?
You cannot tell reliably without testing. Both cause humming and failure to start. A technician measures the capacitor and the compressor windings, and the readings make the answer clear within minutes.
What is a hard-start kit and does it help?
It is an added start capacitor and relay that give the compressor extra torque at startup. It can help an aging but mechanically healthy compressor that struggles to start. It will not revive a compressor with damaged windings or internal mechanical failure.
Is it worth replacing just the compressor?
It can be on a newer system still under parts warranty, with good coils and a current refrigerant. On an older system, the total cost of compressor, refrigerant, drier and labor often makes a new matched outdoor unit the better value.
Why did my compressor fail?
Common causes are low refrigerant, dirty condenser coils, electrical problems such as weak capacitors or loose connections, and liquid refrigerant returning from the indoor coil. Identifying the cause matters so the replacement does not suffer the same fate.
Can a compressor be repaired internally?
Residential compressors are hermetically sealed, so internal repairs are not practical. Repair work focuses on the electrical and control components around the compressor, and a failed compressor is replaced as a unit.
Related cooling pages
The compressor sits at the center of these related repair and replacement services.