How long should I wait after changing the filter before my AC starts cooling again?

Give the system 20–30 minutes after changing a severely clogged filter. If the evaporator coil froze due to restricted airflow, you may need to let it thaw for 2–4 hours with the system in fan-only mode before full cooling returns.

My AC runs constantly but can't reach the thermostat setpoint on hot days. Is that normal in Fairfax?

On extreme heat days, it's normal for a correctly sized AC to run nearly continuously. If it's running non-stop on a moderate summer day and still can't catch up, you likely have an airflow problem, a refrigerant issue, or a system sizing problem.

Should I turn my AC off if it's not cooling, or keep running it?

If you suspect a frozen coil, switch to fan-only mode or turn the system off. For other issues like a dirty filter or dirty condenser, you can continue running while you address the fix. When uncertain, fan-only is the safest setting.

How do I verify an HVAC contractor is licensed in Virginia?

Virginia requires all HVAC contractors to hold an active license through the Virginia Department of Professional and Occupational Regulation (DPOR). You can verify any contractor's license status on their website.

Is a heat pump a good replacement for a failing AC in Northern Virginia?

Yes — modern variable-speed heat pumps are well-suited to NoVA's Climate Zone 4A. They qualify for the largest federal tax credits available in 2026 and Dominion Energy offers rebates for qualifying installations.

Back to BlogAir Conditioning · Fairfax, VA · Northern Virginia

AC Capacitor Failure: Symptoms, Cost & When to Call an HVAC Technician

AC humming but not starting on a hot Fairfax afternoon? A small, inexpensive part called the capacitor is behind a large share of summer repair calls. The eight symptoms of failure, what drives replacement cost, why it is never a DIY repair, and how a cheap fix prevents a compressor failure.

September 26, 2026 By the Fairfax Mechanical Team 9 min read

Short answer: The capacitor is one of the most common parts to fail in an air conditioner, and it is usually an affordable, quick fix. It is not a do-it-yourself repair, because capacitors store a dangerous electrical charge even when the power is off. A cheap, timely capacitor replacement can also prevent a very expensive compressor failure.

What does an AC capacitor actually do?

If your air conditioner suddenly will not start on a hot Fairfax afternoon, or it hums but the outdoor fan just sits there, there is a good chance a small, inexpensive component called the capacitor has failed.

Think of the capacitor as the battery that gives your AC its kick. Your air conditioner has motors that need a substantial jolt of electrical energy to start up, the compressor and the fans, and a smaller, steady supply to keep running. The capacitor stores and delivers that energy. There are typically start capacitors, which provide the initial startup jolt, and run capacitors, which keep motors turning smoothly, and some systems use a combined dual capacitor.

Without a healthy capacitor, the motors cannot start or cannot run properly, so even though the rest of the system is fine, the AC fails to cool. It is a small part with an outsized role.

Why capacitors fail so often in Fairfax

Capacitors are especially vulnerable to heat, and Fairfax's hot, humid summers are hard on them. Over years of operation, the intense summer heat gradually degrades the capacitor until it can no longer hold a proper charge. This is exactly why capacitors so often fail during a heat wave, the very time you are relying on your AC most and demand for repairs across Northern Virginia is highest. Age, power surges, and simple wear also contribute.

Symptoms of a failing or failed AC capacitor

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The AC will not turn on at allEven though the unit has power and the thermostat is calling for cooling.
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The unit hums but does not startThe motor is trying to run but lacks the jolt to get going.
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The outdoor fan is not spinning while the unit humsA classic sign of a bad capacitor, though you should still call a professional.
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Hard startingThe system struggles or hesitates to start, or starts and then shuts off quickly.
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Weak coolingThe AC runs but cannot keep up with the heat.
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A clicking sound from the outdoor unitAs it tries and fails to start.
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A swollen, bulging, or leaking capacitorA clear visual sign of failure a technician will spot.
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Higher energy billsAs the straining system works inefficiently.

Do not test it yourself

A capacitor holds a charge even with the power off. It is not a part to handle without training and the right tools, no matter how simple it looks in a video.

What determines AC capacitor replacement cost

Capacitor replacement is generally one of the more affordable and common AC repairs. The cost depends on a few factors:

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The type and rating of the capacitorSingle versus dual, and its specific electrical specifications.
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Your specific system and accessibilityHow easily the component can be reached on your unit.
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Emergency vs. scheduled timingA same-day call during a Fairfax heat wave may differ from a routine visit.
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The diagnosticThe symptoms of a bad capacitor overlap with other electrical issues, including a failing contactor, a bad motor, or a compressor problem. A proper diagnosis confirms the capacitor is truly the culprit.

Always ask for a written estimate before the repair, and have the technician confirm the diagnosis.

When to call an HVAC technician, and why it is not DIY

Capacitors store an electrical charge and can deliver a dangerous, even harmful, shock, even when the power to the unit is off. Discharging and replacing a capacitor safely requires proper tools and training, which is why this is firmly a job for a licensed technician, not a weekend project.

Why a licensed technician

✓Safe discharge of the stored charge before handling
✓Confirmation that the capacitor is the real fault
✓A check for whether the failure stressed other components
✓Correct part rating matched to your system
✓A written estimate before any work begins

Why not do it yourself

✕A capacitor holds a charge even when powered off
✕Symptoms overlap with contactor, motor, and compressor faults
✕The wrong rating can damage the motor it protects
✕No way to tell whether the failure damaged something else
✕A misdiagnosis means paying for the wrong part

Call a technician if your AC will not start, hums without starting, the outdoor fan will not spin, or you hear clicking from the outdoor unit. And do not delay: running an AC with a failing capacitor forces the compressor to strain, and the compressor is by far the most expensive part of the system.

Prevention

Capacitors wear out eventually, but you can extend their life and catch a weak one early with annual AC maintenance. A tune-up tests the capacitor's charge and flags one that is degrading before it fails on the hottest day. It is part of why maintenance pays for itself.

“A capacitor is a small part that stops the whole system, and it is usually one of the cheapest repairs we make. The expensive mistake is waiting, because a strained compressor is a completely different conversation.”Fairfax Mechanical Technical Team, Fairfax, VA

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Frequently Asked Questions

Is a capacitor an expensive repair?

No. Capacitor replacement is typically one of the more affordable and common AC repairs. It is a small part, and for a technician it is a relatively quick job. Cost depends on the type and rating your system uses, how accessible the component is, and whether the visit is an emergency or a scheduled one.

Can I replace the capacitor myself?

No. Capacitors store an electrical charge and can deliver a dangerous, even harmful, shock even when the power to the unit is off. Discharging and replacing one safely requires proper tools and training, which is why this is firmly a job for a licensed technician.

Why did my capacitor fail during the heat wave?

Heat degrades capacitors over time, so they often give out under peak summer demand. Fairfax's hot, humid summers are hard on them, and capacitors so often fail during a heat wave, the very time you are relying on your AC most.

What are the symptoms of a bad AC capacitor?

The AC will not turn on at all despite having power, the unit hums but does not start, the outdoor fan is not spinning while the unit hums, the system struggles or hesitates to start, cooling is weak, you hear clicking from the outdoor unit, or you see a swollen, bulging, or leaking capacitor.

Why choose Fairfax Mechanical for capacitor replacement?

We are an experienced, licensed Virginia HVAC contractor, and we quickly and safely diagnose and replace failed capacitors, confirming the capacitor is the real problem and not a symptom of something larger, so you are not paying for the wrong fix. We offer same-day AC repair across Fairfax and all of Northern Virginia.

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