If your AC's outdoor fan is spinning but the compressor isn't turning on, the system is running without actually cooling anything. The usual causes are a failing capacitor, a bad contactor, or, on the hottest days, a compressor that's overheated and shut itself down on purpose until it cools.

That last one surprises a lot of people who've never lived somewhere hotter than 100°F. Walk through what you're seeing before calling anyone, because the symptom points pretty clearly toward a short list of causes.

The fan-runs-but-compressor-doesn't scenario

Walk out to the outdoor unit while the system is calling for cooling. If the fan on top is spinning but you don't hear the low hum or vibration of the compressor running, and the lines aren't getting cold, the compressor itself isn't engaging even though the rest of the unit has power. This is different from the whole unit being dead, and it narrows things down fast.

Capacitor failure — again

The capacitor shows up in almost every AC troubleshooting guide for a reason: it's one of the most failure-prone parts in the whole system, and Arizona heat shortens its life faster than almost anywhere else in the country. A dual-run capacitor gives both the fan motor and the compressor the electrical push they need to start. When it weakens, sometimes the fan still gets enough of a push to start (fans need less starting torque) while the compressor doesn't, which produces exactly this symptom: fan spinning, compressor silent. We cover capacitor symptoms in more depth in AC fan not spinning, since a fully failed capacitor often takes both down together.

A weak capacitor sometimes shows a faint humming from the compressor as it tries and fails to start. If you hear that hum, don't let it keep trying repeatedly — that struggle-to-start cycle stresses the motor windings even when it never fully starts.

Contactor problems

The contactor is the relay that switches power to the compressor and outdoor fan when the thermostat calls for cooling. Pitting on the contactor's contacts, from years of arcing every time it engages, can eventually prevent it from making a clean connection to the compressor circuit while still allowing power through to the fan on a separate leg. A stuck or burned contactor is a common, relatively inexpensive fix, but diagnosing it means testing voltage with the panel open, live power present, which is technician work.

Thermal overload — a real Arizona phenomenon

This cause is specific to extreme-heat climates like Arizona. Compressors have an internal thermal overload protector that shuts the compressor down if it gets too hot, protecting the motor windings from heat damage. On a 115°F day, especially in the afternoon after the unit has been running for hours, the compressor itself can heat-soak enough to trip that protector even when nothing is actually broken. The compressor then won't restart until it cools down, sometimes 20-30 minutes, sometimes longer if the unit is in direct sun with poor airflow around the cabinet.

This is a different situation from a mechanical failure, and it's worth ruling out before assuming the worst: if the compressor was working that morning, quit in the heat of the afternoon, and the outdoor unit is sitting in full sun with the fan running fine, give it time to cool and see if it restarts once temperatures drop in the evening. If it restarts fine at night but keeps quitting on hot afternoons, that's a system running right at the edge of its capacity, worth having evaluated, but not necessarily broken.

Hard-start kits

A hard-start kit adds extra starting torque to help a struggling compressor overcome the initial resistance of starting, which can mask a weak capacitor or a compressor that's beginning to wear internally. It's a legitimate fix in some cases, especially on older systems or in extreme heat, but it's a patch, not a cure, if the underlying cause is a compressor nearing end of life. A technician should explain which situation you're in before installing one, since a hard-start kit on a compressor that's actually failing buys you time. It's not a permanent fix.

Repair vs. replace when the compressor itself is dead

If a technician confirms the compressor's windings have failed internally, rather than a capacitor, contactor, or thermal trip being the cause, you're at a real decision point. A compressor replacement is one of the most expensive single repairs on an AC system, and on a unit already past 10-12 years old, many contractors will walk you through the math on repairing versus replacing the whole system, since a new compressor on an old system doesn't extend the life of everything else that's aged alongside it. Ask for both numbers before deciding, and get a second quote if the difference between repair and replacement is close.

What a technician checks

A technician diagnosing a silent compressor will test the capacitor's microfarad rating, check the contactor for continuity and pitting, measure compressor winding resistance to rule out a short or open circuit, and check whether the thermal overload has tripped and why. Expect a standard diagnostic visit in the $75-$150 range, with capacitor or contactor repairs typically modest, and compressor replacement quoted as a significant separate cost once confirmed.

Any contractor working on your system should carry an active Arizona ROC license, verifiable at azroc.gov before you schedule anything.

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For the outdoor fan not spinning at all, see AC fan not spinning. For other symptoms, our AC troubleshooting hub sorts through the common ones.

Common questions

Why is my AC fan running but the compressor won't start?

The most common cause is a weakening capacitor that still gives the fan enough starting torque but not the compressor. A failing contactor or, on extremely hot days, a heat-tripped thermal overload protector are the other two likely causes. A technician can test which one with the panel open.

Can extreme heat really stop a compressor from starting?

Yes. Compressors have a built-in thermal overload that shuts them down if internal temperatures climb too high, which can happen on 115°F days after hours of runtime. The compressor usually restarts on its own once it cools, often in the evening, without any repair needed.

Is a hard-start kit a permanent fix for a compressor that won't start?

Not always. It gives the compressor extra torque to overcome starting resistance, which helps a weak but otherwise sound compressor, or an older system in extreme heat. If the compressor itself is failing internally, a hard-start kit buys time rather than solving the underlying problem.

How do I know if I need a new compressor or a new AC system?

That depends on the system's age, the cost of the compressor repair, and the condition of everything else — coils, ductwork, efficiency rating. On units past 10-12 years old, many contractors will walk you through repair-versus-replace numbers side by side. Get that comparison in writing before deciding.