You walk out to the condenser on a hot afternoon and hear it: a low hum or buzz from the outdoor unit, maybe a click every few minutes, but the compressor never spins up. Indoors the blower runs and the vents blow room-temperature air. On the worst days it hums for a few seconds and the breaker trips.
The compressor is the heart of an air conditioner — the pump that moves refrigerant, and the most expensive part in the system. That makes this the page where repair-versus-replace actually matters. The good news: most "compressor not working" calls are not a dead compressor. They're a cheap electrical part that can't get the compressor moving.
How it usually shows up:
- The outdoor unit hums or buzzes but nothing spins.
- You hear a click, a hum for a second or two, then silence — over and over.
- The compressor runs a few minutes, shuts off on its own, and won't restart until it's cooled.
- The AC breaker trips every time the system tries to start.
Shut it off first
If the unit hums and the compressor won't start, turn the system off at the thermostat and at the outdoor disconnect or breaker. A compressor that's energized but not turning draws locked-rotor current — many times its normal running amps — and every attempt heats the motor windings. Repeated locked-rotor starts are one of the fastest ways to turn a cheap capacitor repair into a compressor replacement, and they're what's tripping your breaker. Leave it off until a tech can look at it; you're not losing cooling you would have had anyway.
Why the compressor won't start
Roughly in the order we actually find them:
- A failed run or start capacitor. By far the most common cause, and the cheapest fix. The capacitor gives the compressor and condenser fan the jolt they need to get moving; when it weakens, you get the classic symptom of a compressor that hums but won't turn over.
- A bad contactor. The relay that switches line voltage to the outdoor unit. Pitted, welded, or chattering contacts mean the compressor never gets clean power.
- Overheating from a dirty condenser coil. A coil packed with dust or blocked by shrubs can't reject heat, so the compressor overheats and its internal overload cuts it out — it restarts only after cooling, then does it again.
- Low refrigerant from a leak. Refrigerant also cools the compressor motor. An undercharged system runs hot, trips out on overload, and wears the compressor out early.
- A tripped internal overload. The compressor's thermal protector doing its job — a symptom, not the root cause.
- Electrical supply or start-assist problems. Loose lugs, a failed disconnect, low incoming voltage, or an older compressor that has gotten hard to start and needs a hard-start kit.
- A locked or seized compressor. The genuine mechanical failure — real, but the least common on this list, which is why a good tech rules out everything above first.
The usual fix
Our tech starts with the electrical side, because that's where the answer usually is: test the capacitor under load, inspect the contactor, and check incoming voltage and startup amp draw. Locked-rotor amps tell us quickly whether the motor is trying and failing or genuinely seized. From there we check the condenser coil for fouling and read refrigerant charge and pressures. Only when the electrical and charge sides check out do we test the compressor windings and call it a failed compressor. You get a firm quote before any part goes in.
Repair or replace — the honest version
If it really is the compressor, the math changes. A compressor replacement is the most expensive repair on an air conditioner, and there are cases where putting one into an old system is money poorly spent:
- A system over about 12–15 years old. You'd be installing the priciest part into a unit whose coils, fan motor, and controls are near the end of their service life too.
- An R-22 system. R-22 was phased out of production and imports; it's expensive and hard to source, and a major sealed-system repair on one rarely pencils out.
- A system that's already had significant repairs, or one that's been undersized for years.
A compressor on a newer system, especially one still under a manufacturer parts warranty, is usually worth repairing. We'll give you both numbers and let you decide — our repair or replace guide walks through how we think about it.
DIY vs. calling a pro — the honest version
Safe to check yourself first (free):
- Shut the system off the moment you hear it hum without starting. That's the most useful thing you can do.
- Reset the breaker once. If it holds, fine; if it trips again, stop. A repeat trip is a fault, not a fluke.
- Clear the condenser of leaves, cottonwood, and weeds, and give it about 2 feet of clearance on all sides. Overheating is a real cause of compressor cutouts.
- Change a dirty filter and make sure return grilles are clear.
Call a pro when:
- The unit hums, buzzes, or clicks but the compressor won't start — don't keep trying it.
- The breaker trips on startup.
- The compressor runs a few minutes and cuts out repeatedly.
- You suspect a capacitor, contactor, or anything involving the refrigerant circuit.
Capacitors store a dangerous charge even with the power off, the contactor carries line voltage, and refrigerant work is licensed-tech only (EPA 608). This is not a DIY repair.
What it costs in the Bay Area
The spread here is wide, which is exactly why diagnosis matters. In our pricing, a capacitor runs about $120–$350 and a contactor $120–$320 — the inexpensive fixes that solve most humming-compressor calls. A compressor replacement is the high end at $900–$1,600. We charge a diagnostic fee that's credited toward the repair when you approve the work, and you'll always see a flat, upfront price before we start — including, when it's relevant, an honest comparison against replacing the system.
Get your AC running again
Triton repairs central AC, heat pumps, and mini-splits across the Tri-Valley, East Bay, and Peninsula — Pleasanton, Livermore, Dublin, San Ramon, and Hayward, where inland summers push compressors hardest. Same-week (often same-day) appointments and licensed techs.
Book online or call (888) 993-5699. We'll find out whether it's a capacitor or the compressor before you spend anything on the wrong one.
Related pages
- HVAC Repair in the SF Bay Area — our full heating & cooling hub
- AC Condenser Fan Not Spinning (Just Humming) — the fan-side capacitor fix
- AC Won't Turn On / Outside Unit Not Running — total no-start diagnosis
- HVAC Repair or Replace? — how we run the numbers
- HVAC & AC Repair in Pleasanton — Tri-Valley service area
Frequently asked questions
Why is my AC compressor humming but not starting? That hum is the compressor trying to turn and failing, and the most common reason by far is a failed run or start capacitor — an inexpensive part. A bad contactor, an overheated compressor, or a seized compressor can cause it too. Turn the system off right away: repeated attempts draw locked-rotor current that can destroy the compressor.
Should I keep trying to start my AC if the compressor hums? No. Every attempt pulls several times normal current through the motor windings, which is how a cheap capacitor repair turns into a compressor replacement. It's also what trips your breaker.
Is a humming compressor always a bad compressor? No, and that's the important part. A seized compressor is the least common cause. Techs test the capacitor, contactor, voltage, and startup amp draw first, then check the condenser coil and refrigerant charge. Only after all of that checks out is the compressor itself condemned.
Is it worth replacing an AC compressor? It depends on the system. On a newer unit, especially one with a compressor still under a manufacturer parts warranty, repair usually makes sense. On a system over roughly 12 to 15 years old, or an older R-22 system, putting the most expensive part into aging equipment often favors replacement. We give you both numbers before you decide.
How much does an AC compressor repair cost in the Bay Area? It depends on the actual fault: a capacitor is about $120–$350 and a contactor $120–$320, while a compressor replacement is the high end at $900–$1,600. Most humming-compressor calls turn out to be the cheap parts. We credit the diagnostic fee toward the repair and give you a firm price before any work begins.