Is it still under warranty?
Compressors frequently carry longer parts coverage than the rest of the system. We check registration status before quoting.
How a compressor is actually tested
Four things that imitate a failed compressor, and how to tell them apart
The compressor is the single most expensive component in a cooling system, which makes it the one worth being slowest to condemn. It is also the component whose failure symptoms are shared by several much cheaper faults, so an honest diagnosis is mostly a process of elimination carried out with a meter rather than an opinion formed from a noise.
The first candidate is the run capacitor. A compressor motor needs a phase shifted current to develop starting torque, and the capacitor provides it. As capacitance falls, available torque falls with it, and there comes a point where the motor energises, cannot overcome the pressure differential across it, draws locked rotor current and trips its internal overload. The homeowner hears a hum and a click. The part costs very little and reads out in seconds on a meter, which is why it is checked before anything else.
The second is the contactor. Its contact faces carry the full load current and they erode, arc and pit over years of switching. A pitted contactor adds resistance in series with the motor, so the voltage actually arriving at the compressor terminals sags under load even though the supply measured at the disconnect looks perfect. Measuring voltage at the terminals while the motor attempts to start is what separates this from a motor fault, and it is a measurement plenty of technicians never take.
The third is mechanical resistance elsewhere in the circuit. A compressor that has to start against an abnormally high head pressure, because the condenser fan has failed or the coil is completely blocked, will behave exactly like a weak motor. The fourth is the supply itself, where an undersized conductor, a loose lug or a brownout condition drops voltage below what the motor needs. Only once all four are eliminated does attention move to the compressor, where winding resistance between each terminal pair and insulation resistance to the shell are measured directly. Those two tests distinguish an open winding, a shorted winding and a motor that has gone to ground, and they produce numbers rather than impressions.
| Test | What it measures | What it rules in or out |
|---|---|---|
| Capacitance against nameplate | Actual microfarads delivered by the run capacitor | Loss of starting torque, the most common cause of a hum and click |
| Voltage at compressor terminals under load | Supply arriving after the contactor | A pitted contactor or an undersized or loose supply conductor |
| Winding resistance, terminal to terminal | Continuity and balance across the motor windings | An open or shorted winding inside the compressor |
| Insulation resistance to shell | Leakage from windings to the compressor body | A motor that has gone to ground, which is unrecoverable |
| Head pressure at start attempt | Pressure the motor must start against | A failed condenser fan or a fully blocked coil imitating a weak motor |
| Oil acidity after failure | Chemical condition of the system oil | Whether the circuit needs flushing before a new compressor goes on |
Where the compressor genuinely has failed, the next question is contamination. A motor that burned out has released acid into the oil and therefore into the whole sealed system. Fitting a new compressor onto a contaminated circuit without a thorough clean up is how a fifteen year machine becomes a three year one, so the oil is tested and the system flushed and filtered accordingly.
It is also worth understanding why compressors fail, because the cause usually predicts whether a replacement will last. Liquid refrigerant returning down the suction line washes oil off the bearing surfaces and eventually damages the valves. Persistently low airflow across the indoor coil produces exactly that condition. Overheating from a blocked condenser or a failed fan cooks the windings from the inside. In every one of those cases the compressor is the casualty rather than the culprit, and replacing it without correcting the airflow or the condenser guarantees the same outcome again on a shorter timescale.
Decision framework
Six questions that decide repair, component, or replacement
Once the compressor really is the fault, the decision stops being technical and starts being arithmetic. These are the inputs.
How old is the rest of the machine?
A condenser coil and fan motor of the same age have the same remaining life, which changes the arithmetic.
What refrigerant does it use?
Equipment on a refrigerant being phased down faces rising service costs for the remainder of its life.
Was there a burnout?
Contamination adds flush, filter drier and follow up cost, and raises the risk on any repair.
Is the indoor coil a match?
Replacing only the outdoor half onto an untested pairing loses the published rating entirely.
What does replacement actually cost?
The comparison is only honest if both figures are on the table at the same time.
Scope of the diagnosis
What happens before anything is condemned
Every one of these is performed before the word compressor appears on a quote.
Capacitor measuredActual capacitance against the printed rating, not a visual check for bulging.
Contactor inspectedFaces examined and voltage measured at the compressor terminals while it attempts to start.
Windings testedResistance across each terminal pair and insulation resistance to the shell, both recorded.
Head pressure verifiedSo a failed condenser fan is not mistaken for a weak motor.
Warranty status checkedBefore quoting, because the manufacturer may still owe you the part.
Three figures givenComponent repair, compressor replacement and full system replacement, with the reasoning for each.
Customer testimonies
Feedback on compressor diagnosis
I had a quote for a whole new system. The actual fault was a contactor with burnt faces and a capacitor reading low.
They tested the windings in front of me and wrote the resistances down. The compressor was genuinely dead, but at least I could see why.
Checked the warranty before quoting and the compressor was still covered. That saved me the largest part of the bill.
Compressor questions
Diagnosis, hard start kits, burnout and when replacement wins
The unit hums and then clicks off. Is the compressor finished?
Usually not. That pattern is the internal overload protector interrupting the circuit after the motor drew locked rotor current, which means it tried to start and could not develop enough torque. A weak run capacitor is the most frequent reason, followed by voltage sag caused by a pitted contactor.
What matters is that you stop trying. Each attempt heats the windings substantially, and enough repetitions will turn a cheap electrical repair into a genuine motor failure. Switch the system off at the thermostat and leave the breaker alone until someone has measured it.
Is a hard start kit a proper fix?
It is a legitimate component in the right circumstances and a way of hiding a problem in the wrong ones. A start capacitor and potential relay increase starting torque, which genuinely helps on long line sets, on systems with a history of marginal supply voltage, and on some older single phase compressors.
Fitting one to mask a compressor with degrading windings just postpones the failure and removes the early warning. We will fit one where the measurements support it and say so when it would only be papering over a motor that is on its way out.
What is a burnout and why does it matter so much?
A burnout is a compressor motor failure in which the windings overheat badly enough to break down the insulation and the refrigerant oil. The chemical result is acid, and because the motor sits inside the sealed refrigerant circuit, that acid is now distributed throughout the system.
Installing a new compressor onto an uncleaned circuit puts the replacement straight into an acidic environment, and it will fail early. Proper recovery involves testing the oil, flushing or replacing contaminated components, fitting a suction line filter drier and, on a severe burnout, replacing the line set. Skipping that is the single most common reason a replacement compressor does not last.
When is replacing the whole system the better answer?
When warranty coverage has expired, the coil and fan carry identical service history, and the system runs a refrigerant whose supply is tightening year on year. Those three together mean a large spend puts one new part inside an old machine, with the next failure likely within a few seasons.
Replacement is the weaker argument when the equipment is under about eight years old, the coil and fan are sound, and particularly when the compressor is still covered by the manufacturer. We will put both figures in front of you with the reasoning and then leave the decision alone.
Related services
What a compressor diagnosis usually connects to
Compressor work sits alongside the rest of the cooling catalog, and the decision often turns into a replacement conversation.
Emergency AC Repair
AC Installation
Central Air Installation
AC Tune-Up
Ductless Mini Split Installation
Told your compressor is finished? Get it measured
The electrical tests that separate a dead compressor from a dead capacitor take under an hour, and in Yorkville, NY they change the answer often enough to be worth doing before you spend anything.
Diagnostic and repair figures are both quoted before work starts, so the decision stays yours.