A failed capacitor is one of the most common reasons a central air conditioner stops cooling in the summer. It's also one of the most cost-effective AC repairs — the part costs $15–$60 and replacement is straightforward for someone comfortable working around electricity. Understanding capacitor failure helps you diagnose the problem quickly and decide whether to DIY or call a tech.
What Does an AC Capacitor Do?
An air conditioner has two types of capacitors, and both are critical:
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Start capacitor: Provides the extra electrical "kick" needed to start the compressor or fan motor. It only operates during the brief startup period. Run capacitor: Maintains a phase-shifted electrical current that keeps the motor running efficiently after startup. It operates continuously while the compressor or fan is running. Many modern AC units use a dual run capacitor — a single cylindrical component that serves both the compressor and the condenser fan motor. Dual run capacitors are rated in microfarads (μF) with two values printed on them (e.g., 45/5 μF) — one for the compressor and one for the fan.
Signs of a Failed AC Capacitor
AC won't start: The unit tries to start (you can hear it hum) but the compressor or fan doesn't spin up. This is the classic capacitor failure symptom — the motor gets stuck because it can't get the starting current it needs.
AC starts but quickly shuts off: The unit starts briefly and then trips on thermal overload. A weak capacitor means the motor draws more current than normal to run, causing it to overheat quickly.
Fan spinning slowly or not at all: If the condenser fan blade is turning very slowly or you can give it a manual spin to get it going, the run capacitor is likely failing (dangerous to test this way — always shut off power first).
Humming noise with no cooling: The compressor tries to start, creates a hum from the electrical load, but doesn't actually spin.
Bulging or leaking capacitor: A visually failed capacitor — the top is domed instead of flat, or there's brown residue on the outside — is clearly bad and needs replacement.
How to Diagnose a Capacitor with a Multimeter
A multimeter with capacitance testing mode (common on mid-range multimeters) can confirm capacitor failure:
1. Shut off power at the disconnect box. 2. Open the condenser unit's electrical panel (usually a small removable panel on the side of the unit). 3. Discharge the capacitor using an insulated screwdriver (briefly short across the terminals through a resistor — or let it sit for 5+ minutes after power off). 4. Disconnect the capacitor leads (photograph them first for reference). 5. Test microfarad value with the multimeter in capacitance mode. 6. Compare to the rated μF value on the capacitor label — if more than 6% below the rated value, the capacitor is weak and should be replaced.
Important safety note: Capacitors store high-voltage charge even after power is disconnected. Always discharge before handling. A charged capacitor can deliver a dangerous or fatal shock.
Replacement Cost
Dual run capacitor (DIY): $15–$60 at HVAC supply stores or online. Capacitor replacement by HVAC tech: typically $150–$350 total (labor + part markup). The large difference makes this a worthwhile DIY repair for homeowners comfortable with basic electrical work who are careful about safety protocols.
Buying the Right Replacement Capacitor
Match the microfarad (μF) rating exactly — or within 5–10% for run capacitors if the exact value isn't available. Voltage rating must be equal to or higher than the original. Same form factor (round/oval, same terminal type). Never substitute a start capacitor for a run capacitor — they're different designs for different purposes.
When Capacitor Failure Means AC Replacement
A capacitor failure on a unit that's 3–10 years old is a normal repair — replace the capacitor and move on. If your AC is 15+ years old and has had multiple capacitor failures, failing capacitors often signal that the compressor and motors are operating under stress. A compressor failure on an old unit typically means full system replacement.
If you're at that decision point, Furnace Direct carries Goodman central AC systems at factory-direct wholesale pricing. Browse replacement options at furnace.direct/collections/cooling.
Related reading: How to Clean AC Coils | AC Startup Checklist | Signs Your AC Needs Replacement
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