Bad AC compressor: symptoms and your options
The signs of a bad AC compressor are warm air with the outdoor fan running, a breaker that trips when the unit starts, loud clattering or grinding, and hard, shuddering starts. Several cheap parts cause the same symptoms, so the compressor has to be confirmed with electrical and pressure tests. If it has failed, you choose between a new compressor, a new outdoor unit or a new system.
Safety first. If the outdoor unit bangs, grinds or trips the breaker, turn the system off and leave it off. Compressor testing involves high voltage and refrigerant and is strictly a job for a licensed, EPA-certified technician.
Check these first
0 of 6 checkedStill not working after these? That is the point to bring in a licensed technician.
What the compressor does and how it fails
The compressor is the pump in the outdoor unit that moves refrigerant through the system. It is a sealed, welded shell with an electric motor inside, so it cannot be opened and repaired. When it fails, it is replaced.
It fails in three ways. Electrically, the motor windings short to each other or to the shell, or they burn open. Mechanically, internal parts wear, break or seize. And it can simply stop pumping well: it runs, draws power and moves little refrigerant.
Compressors seldom die of old age alone. The usual killers are years of running with a low refrigerant charge, dirty coils, a weak capacitor, or liquid refrigerant returning to it from a frozen coil.
Symptoms, and what else causes them
| Symptom | If it is the compressor | Cheaper causes to rule out first |
|---|---|---|
| Fan runs, no cooling, no compressor hum | Open winding or seized compressor | Failed capacitor, worn contactor, overheated compressor waiting to reset |
| Breaker trips at startup | Winding shorted to ground, locked rotor | Weak capacitor, damaged wiring, weak breaker |
| Clattering, banging or grinding | Broken internal parts | Loose fan blade, debris, loose mounting hardware |
| Hard, shuddering starts | Worn, tight compressor | Weak capacitor, low voltage |
| Runs but cools poorly | Worn compressor that no longer builds pressure | Low refrigerant, dirty coils, airflow problems |
You cannot tell these apart from the yard. Many compressors called dead over the phone turn out to need a capacitor. Expect a real diagnosis before you accept the verdict.
How a technician confirms it
The tests are specific. The technician measures resistance across the windings and from each winding to ground, checks the capacitor and contactor, measures voltage at the unit, and reads current as the compressor tries to start. If it runs, they compare the suction and discharge pressures. A compressor that runs with pressures nearly equal is no longer pumping.
Ask for the readings and the reason for failure in writing. If the cause was a leak or a dirty system and it is not fixed, a new compressor will fail the same way.
Your options
- Replace the compressor. The most labor-heavy repair on an AC. It includes recovering the refrigerant, brazing in the new compressor, a new filter drier, a deep vacuum and a full recharge. It fits best when the compressor is still under the manufacturer's parts warranty. Labor and refrigerant are usually not covered.
- Replace the outdoor unit. A new condensing unit comes with a new compressor, coil and fan. It has to be matched to your indoor coil and refrigerant.
- Replace the whole system. The sensible path when the system is old, uses a refrigerant that is no longer produced, or the indoor coil is also in poor shape.
The compressor replacement cost guide explains what goes into the repair price. The guide on whether to repair or replace your air conditioner walks through the decision using age, warranty, refrigerant type and repair history. The AC repair overview covers the smaller faults worth ruling out first.
Common questions
What are the first signs of AC compressor failure?
Early signs are harder starts, a louder or rougher sound from the outdoor unit, longer run times with weaker cooling, and an occasional tripped breaker. These also fit a weak capacitor or a dirty coil, which are cheap to fix, so have the unit tested when you first notice them.
Can an AC compressor be repaired instead of replaced?
The compressor itself is a sealed, welded unit and cannot be opened in the field. What can be repaired are the parts around it: the capacitor, the contactor, wiring and a hard-start kit for a compressor that is slow to start. If the compressor is shorted, seized or not pumping, it is replaced.
My AC compressor is not working but the fan is running. Is it dead?
Not necessarily. A failed capacitor, a worn contactor or a compressor that overheated and is waiting to reset all produce a running fan with no cooling. Turn the system off, let it cool, and have a technician test the capacitor and the compressor windings before you assume the worst.
Is the compressor covered by warranty?
Often the part is, for a period set by the manufacturer and sometimes conditional on registration. Labor, refrigerant and related parts are usually separate. Look up your model and serial number with the manufacturer or ask the contractor to check before you approve a quote.
Related guides
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