Your outdoor AC unit is humming but the compressor won’t start, or the fan is spinning slowly, or the system trips the breaker every time it tries to cool. In almost all of these cases, the capacitor is the first thing to check.
It’s the single most replaced component in a residential AC system, it costs $10–40 for the part, and it’s a repair most homeowners can do safely — as long as you know and follow the one rule: the capacitor stores lethal voltage even when the power is off. Discharge it before touching anything.
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What the capacitor does
An AC capacitor provides the extra electrical “kick” needed to start and run the compressor and fan motors. Standard induction motors can’t start on their own — they need a phase-shifted voltage to get the rotor spinning. The capacitor stores a charge and releases it at the right moment.
Run capacitor: Maintains current to the motor during normal operation. Keeps the motor running efficiently.
Start capacitor: Provides a brief high-capacitance boost at startup only, then drops out of the circuit. Less common in residential systems — most home AC units use a run capacitor for both functions.
Dual-run capacitor: One can with two capacitance ratings — typically labeled as something like 45/5 MFD or 35/5 MFD. The larger value (45 MFD) serves the compressor; the smaller (5 MFD) serves the condenser fan motor. Three terminals: HERM (compressor), FAN, COM (common). This is what you’ll find on most residential units.
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Symptoms of a bad HVAC capacitor
Compressor hums but won’t start. The most common symptom. You hear the outdoor unit trying to start, the fan may or may not spin, but the compressor is locked up. The unit usually trips the breaker or cycles the safety switch after a few seconds.
Fan spins slowly or barely moves. If the fan-side of the capacitor is failing while the compressor side is still marginal, you’ll see the fan start slowly or spin at low RPM instead of its normal high-speed rotation. The unit may run but cool poorly.
Breaker trips on startup. A compressor trying to start without adequate capacitor support draws a high inrush current and can trip the breaker repeatedly. If the breaker trips once and holds on the second attempt, the capacitor is likely on its way out.
Unit shuts off prematurely on hot days. Marginal capacitors that test in tolerance at room temperature can fail under heat load. The unit runs for 15–20 minutes, then shuts off and won’t restart until it cools down.
Swollen or bulging top. A visual sign: look at the top of the capacitor (usually a silver or blue cylinder mounted inside the condenser cabinet). If the top is rounded or visibly swollen instead of flat, the capacitor has failed internally and needs replacement regardless of how it tests.
Unit won’t start at all, no hum. If the capacitor shorts internally rather than weakening, the contactor may not close or the whole circuit may fail. Less common than the humming symptom.
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⚠️ Safety: discharge the capacitor before touching it
This is not optional and not a formality. A capacitor holds a DC charge — typically 300–400V — for hours after the power is disconnected. That’s enough to kill. The capacitor is fully charged from its last run cycle and will stay charged whether the power has been off for 10 minutes or 10 hours.
Before touching any wires or terminals inside the condenser cabinet:
- Turn off the disconnect switch next to the outdoor unit (the pull-out block or switch in the grey box on the wall).
- Turn off the circuit breaker for the AC at your main panel.
- Wait 30 seconds, then discharge:
– Option A (easiest): Touch the tip of an insulated screwdriver across the HERM terminal and the COM terminal simultaneously. You’ll see a spark — that’s normal. Repeat for FAN to COM. Don’t touch the metal shaft. – Option B (safest): Use a 20,000-ohm (20kΩ), 5-watt resistor. Hold one probe to HERM, one to COM for 5 seconds. Repeat for FAN to COM. No spark, no risk.
- After discharge, verify with a multimeter set to DC volts: the reading across HERM–COM and FAN–COM should be near zero.
If you’re not comfortable with this procedure, stop here. A capacitor replacement is a $150–300 service call. That’s money well spent if you’re not confident with the discharge step.
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How to test a capacitor
You need a multimeter with a capacitance function (the MFD or μF setting). Most digital multimeters $30 and up have this. Dedicated capacitor testers from HVAC suppliers are faster but not required.
1. Discharge the capacitor (see above — this must happen before any probing).
2. Disconnect the wires from the capacitor terminals. Label or photograph which wire goes to which terminal before removing.
3. Set your multimeter to capacitance (MFD / μF mode). Touch the probes to HERM and COM. Read the value. Then test FAN and COM.
4. Compare to the rated value printed on the label.
- Within ±6% of rated MFD: acceptable.
- Beyond 6%: replace the capacitor.
Example: a capacitor rated at 45/5 MFD:
- Compressor (HERM–COM): acceptable range is 42.3–47.7 MFD
- Fan (FAN–COM): acceptable range is 4.7–5.3 MFD
Most failing capacitors test significantly below rated value — a 45 MFD reading as 32 MFD, or a 5 MFD reading as 1 MFD.
What if you don’t have a multimeter? If the symptoms match (humming, slow fan, breaker trip) and the capacitor has a visibly swollen top, replace it. Don’t test — replace.
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Buying the right replacement capacitor
Match the MFD rating exactly (or as close as available — within the 6% tolerance). You cannot substitute a 40 MFD for a 45 MFD and expect the compressor to run properly.
Match or exceed the voltage rating. HVAC capacitors are rated at either 370V AC or 440V AC (this is their AC voltage rating, separate from the DC charge they hold). A 370V cap can be replaced with a 440V cap — higher voltage rating is fine. A 440V requirement cannot be met with a 370V cap.
Terminals and physical size: dual-run capacitors come in round or oval shapes. Either works electrically; the oval may fit tighter in some cabinets. The terminal configuration (HERM, FAN, COM) is standard across brands.
Where to buy: HVAC supply houses (Johnstone Supply, Ferguson, Winsupply) stock common MFD values same-day. Amazon and Home Depot carry many sizes. Avoid off-brand capacitors with no UL listing — the difference between a $12 quality cap and a $7 no-name is about the same as the cost difference between calling a tech and not calling one again next summer.
Tip: take the old capacitor with you or photograph the label clearly before ordering. The label shows MFD, voltage, and sometimes the OEM part number.
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Replacement: is this a DIY repair?
Yes — for most homeowners who are comfortable with the discharge procedure. The repair is:
- Turn off power at disconnect and breaker.
- Discharge the capacitor.
- Photograph the wiring.
- Remove the wire connectors from the terminals (usually slide-off spade connectors).
- Unscrew the mounting band or bracket holding the capacitor.
- Swap in the new capacitor.
- Reconnect the wires per your photo.
- Restore power and test.
The entire repair takes 20–30 minutes. The part costs $15–40 at an HVAC supply house.
When to stop and call a tech:
- You cannot confirm the discharge is complete and are not comfortable with high-voltage work.
- The compressor still won’t start after replacing a confirmed bad capacitor (the compressor windings may be failed).
- The system trips the breaker immediately after startup — this can indicate a locked rotor or compressor failure, not a capacitor problem.
- You smell burning from inside the cabinet.
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Hard-start kit: when the capacitor is fine but the compressor still struggles
If the compressor is struggling to start — especially in hot weather or after a brief power outage — but the capacitor tests good, a hard-start kit (also called a start capacitor kit or SPP, solid-state potential relay) can help. It adds a large supplemental capacitor in parallel at startup, then drops out once the motor is running.
Hard-start kits are common retrofits on older compressors and on units running near their limits. They extend compressor life by reducing startup current draw. Cost: $25–60 for the kit; 20-minute install.
→ See Hard-Start Kit for AC Compressor: What It Does and Whether You Need One for more.
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Replacement cost
| Approach | Part | Labor | Total |
|---|---|---|---|
| DIY | $15–40 | — | $15–40 |
| HVAC service call | $10–40 | $130–250 | $140–290 |
Most service companies mark up the capacitor to $30–80 and charge a minimum service call fee of $100–150. The labor itself takes them 15 minutes. If you’re comfortable with the safety steps above, this is one of the better DIY candidates in HVAC maintenance.
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When to call a pro
- The compressor still won’t start after replacing the capacitor: possible failed compressor or wiring issue.
- The breaker trips immediately on restart: potential locked-rotor compressor failure or electrical fault — not a capacitor issue.
- Burning smell from the cabinet or visible scorch marks on wiring.
- Any refrigerant-related question (low refrigerant is a separate issue that can mimic some capacitor symptoms) — refrigerant handling requires EPA 608 certification.
- You’re not confident the capacitor is fully discharged before proceeding.
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FAQ
Can I run my AC with a bad capacitor? Not for long. The compressor will try to start and either fail or draw excessive current, potentially burning out the start winding or tripping the breaker repeatedly. Every failed startup attempt strains the compressor. Replace the capacitor before running the system.
How long does an HVAC capacitor last? Typically 10–20 years, but heat accelerates failure. Units in hot climates running long cooling seasons see 10–12 year average life. Units in the South sometimes see 5–8 years on the capacitor before first replacement.
Can a bad capacitor cause high electric bills? Yes — indirectly. A marginal capacitor that keeps the compressor running but with reduced efficiency can cause the system to run longer cycles to reach setpoint. The bigger cost risk is that it fails completely in July.
My unit hums but nothing runs. Is it definitely the capacitor? Very likely, but not certain. A seized compressor makes the same humming sound. Test the capacitor first — it’s the easy/cheap fix. If the capacitor is good and the compressor still won’t start, you may have a failed compressor, which is a $1,500–3,000+ repair and often a replacement decision.
What’s the difference between HERM, FAN, and COM terminals? HERM = compressor (hermetic motor). FAN = condenser fan motor. COM = common ground for both. On a dual-run capacitor, HERM and FAN share the COM terminal.
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Related guides
- AC Not Cooling: Diagnostic Order — if you’ve replaced the capacitor and the system still isn’t cooling, work through the full diagnostic ladder
- AC Outdoor Unit Not Running: What to Check — capacitor failure is the first check when the outdoor unit won’t start
- AC Contactor: Symptoms and Replacement — the other common outdoor-unit failure alongside the capacitor
If you’re deciding whether to approve the repair, see our full breakdown of what the job costs and what a fair invoice looks like.
