Signs of a BAD AC Compressor

Last updated: 2026-09-14

Five signs of a bad AC compressor show up in a home central air system. A car AC compressor is a different part and a different repair, so if that's what sent you here, this article won't help.

• Warm air at the vents while the outdoor unit still runs.
• A hard hum or a clunk at startup instead of a clean start.
• A breaker that trips again after you reset it.
• A system that turns off and on too often.
• Refrigerant that has been added more than once.

Every sign on that list also has a cheaper cause. On our calls, a failed run capacitor usually produces the first three by itself. Three numbers decide it. Compressor amps go against the data plate. That is the metal tag on the compressor listing its rated specs. Capacitor microfarads go against the printed rating. A microfarad is the unit that rates a capacitor's storage capacity. And indoor airflow gets measured in CFM per ton, the cubic feet of air the blower moves per minute, per ton of cooling. Ask for all three in writing before you authorize work.

The Signs of a Bad AC Compressor Homeowners See

Burned contactor contacts are what we check first when a breaker won't hold. Worn compressor and fan controls, though, trace back to something else: a system that turns off and on too frequently, the Department of Energy says. So short cycling can be what wore the controls out.

The symptom tells you where to start measuring, and nothing more. The compressor is the most expensive part we replace in that box. It is also the only one you cannot test once it is gone. The evidence leaves on the truck with it.

Before Anything Else, Shut It Off at the Breaker

Kill power at the disconnect or the breaker before you call anyone, including us. Short cycling is one of the conditions that leads to a lack of lubrication, Copeland's technician training notes say. The notes name five mechanical causes of failure: overheating, oil dilution (liquid refrigerant thinning the oil), and refrigerant floodback (liquid washing back to a running compressor). They also name flooded starts (the compressor starting with liquid refrigerant still inside it) and misassembly. Keep resetting the breaker, and you're stacking more start attempts onto a compressor that's already short on lubrication, which is how a repairable fault turns into a dead one.

Here is the order I work in at the unit. Power off, I read the run capacitor in microfarads. Power on, I clamp an ammeter on the compressor common wire. Then the contactor contacts, then indoor airflow. The refrigerant gauges go on last.

Then stop. You cannot legally check the refrigerant side yourself; EPA Section 608 certification is required of anyone attaching hoses and gauges to a refrigerant circuit. Your standing here doesn't come from a meter. It comes from what you require in writing.

The Three Numbers That Belong on the Invoice

Here is what should be on it. Each number rules something in or out.

1. Compressor amp draw, against the data plate.
2. Run capacitor microfarads, against the printed rating.
3. Indoor airflow in CFM per ton.

Compressor amp draw against the nameplate rating

Measured amps well above the nameplate value point to a failing compressor, DOE's 2012 Measure Guideline says. High amps can also mean an overcharge, or non-condensables (gases like air that never condense). Amps below the listed value point somewhere else. Undercharge, a liquid line restriction, or a failed expansion valve.

The plate carries two markings that matter. RLA is rated load amps, the current the compressor was built to run at. LRA is locked rotor amps, the much higher draw at startup. Ask for both readings: what the meter showed, and what the plate says.

The start components and the contactor contacts

Faulty start components and faulty contactor contacts are ordinary electrical causes of compressor failure, Copeland's notes say. Both take minutes to test. It is the first thing we check before calling anything a compressor problem. A weak capacitor reads below its printed microfarad rating; the contactor is the switch that feeds the outdoor unit, and burned contacts show to the eye. Ask for the measured microfarads against the rating, plus a note on the contacts.

Indoor airflow in CFM per ton

Three thresholds get quoted for CFM per ton, and each one governs something different.

• Below 350 CFM per ton, the standard refrigerant charge tests stop being valid, DOE says.
• Below 200 CFM per ton, the indoor coil can ice (frost forms on the coil and blocks airflow further) and compressor life is shortened. Same guideline.
• At least 300 CFM per ton is what a California HERS rater verifies, whenever refrigerant charge verification applies, per California Title 24, Part 6.

So 350 is a diagnostic limit, 200 is a damage line, and 300 is a code minimum. A dirty filter or a restricted duct run is what we usually find behind numbers that low.

If any of the three readings is missing, hold authorization until it's on paper. That's a normal request, not an accusation: the technician already took the readings to reach the diagnosis, so writing them down costs nothing extra.

Parts That Mimic a Dead Compressor

On our calls, a dead capacitor and a compressor with open windings produce the same three signs you noticed from inside the house. From the yard, they're the same failure. From the meter, they're not close.

Faults also interact. Combinations of faults yield false clues that are very difficult to sort out, DOE warns trained technicians.

If Refrigerant Has Been Added Twice, the Compressor Is the Symptom

Copeland puts it plainly. Open or shorted motor windings do mean the compressor needs replacing, but that electrical failure is often the result of a mechanical failure underneath it. Copeland calls finding that fault root cause. Replace the compressor, leave the cause in place, and you've bought a delay.

I see one pattern often enough to name it: a system topped off season after season with no leak search on record. That's a documentation gap, not proof of anything on its own, and too little refrigerant lowers suction pressure, DOE reports. It ices the evaporator, restricts airflow, and shortens compressor life. An overcharge can slug the compressor with liquid refrigerant instead.

You're not expected to diagnose that yourself. Ask two questions instead. Has anyone added refrigerant to this system before? Was a leak search ever done? Landlords need a third question too: what does this unit's service history show about past refrigerant work? A tenant will rarely know.

Repair or Replace Starts at the Nameplate, Not the Age

Age is the easiest number to find, so it's where most repair-or-replace advice starts, but the data plate is the better place. It gives you the refrigerant type, the tonnage, and the RLA and LRA.

What refrigerant the system holds changes the answer

An R-410A system is still legal to repair, and repair parts stay legal to sell, per EPA's 2023 Technology Transitions fact sheet. R-454B can't be dropped into that system, because EPA's SNAP listing covers new equipment only. The plate changes what a new system will hold; it doesn't block a repair on the one you already have.

The ENERGY STAR test: two or more in the replace column

ENERGY STAR publishes a two-column table for this call. Keep repairing if it's under 10 years old, has a good record, and needs only minor work. Replace if it's over 10, has repeat problems, cycles too much, or needs extensive repairs. Two or more items in the replace column probably means replace, ENERGY STAR concludes.

DOE adds four more: over 15 years old, oversized, compressor amps above rated, or a damaged condenser coil.

DOE also supplies a counterweight: contractors usually swap the whole condensing unit. But a compressor-only repair on an older system with a good coil can cost less than a full swap.

DOE doesn't define good condition, so here's what I look at. Are the fins straight and open, or bent flat and packed with dirt? Is there corrosion eating fins off the tubes? The coil is the part I weigh hardest, because a sound coil is what makes a compressor-only repair worth doing.

What California code adds to a compressor or condenser swap

A compressor or condenser swap is work on the refrigerant side, so it triggers the test. California Title 24 requires HERS refrigerant charge verification for air conditioners in climate zones 2 and 8 through 15. That covers inland San Diego County: Alpine, El Cajon, Lakeside, and Spring Valley. Coastal San Diego is climate zone 7 and isn't on that air conditioner list.

To find your zone, ask the county permit office or the contractor pulling the permit. The HERS rater also confirms at least 300 CFM per ton of airflow. On a full replacement, the installation date sets which efficiency standard California enforces.

Frequently Asked Questions

What are the first signs of a bad AC compressor?

Warm air from the vents while the outdoor unit is still running, a rough hum or clunk when it starts. A breaker that won't stay reset, and a system that cycles on and off too fast. Each one has a cheaper explanation too, which is why you measure before you replace anything.

Can a bad capacitor look like a bad compressor?

Yes, it's the most common mix-up we see. A failing run capacitor and a seized compressor can both show up as warm air, a hum at the outdoor unit, and a breaker that won't hold. A microfarad reading checked against the capacitor's printed rating tells them apart.

Is it safe to keep running the AC if the compressor is failing?

No. Shut it off at the breaker. Copeland's training notes list short cycling among the causes of lubrication loss, along with overheating and flooded starts. Every extra start you force on a struggling compressor works against the part you're trying to save.

Does a compressor replacement in San Diego need a HERS test?

Yes, if the address is in climate zone 2 or zones 8 through 15, which covers inland San Diego County. California requires HERS refrigerant charge verification whenever refrigerant components are installed or altered there. Coastal San Diego sits in climate zone 7 and isn't on that list.

Get the Numbers, Then Make the Call

Shut it off at the breaker. Ask any contractor, ours included, for the three readings in writing, and hold authorization until you have them. Then check the license number at cslb.ca.gov. Ours is CSLB #1154431, and it only counts because you can look it up without us.

For related reading: system diagnosis, preventative maintenance, and full AC replacement.

Everglade sells the compressor repairs and replacements this article covers. Ask us for the same three readings you'd ask anyone else for.

Everglade Heating and Air is owner-operated and NATE-certified, serving all of San Diego County including Alpine, Spring Valley, El Cajon, and the coast. For a free in-home installation estimate, call or text Karam at (619) 792-8289.