Why is my AC condenser humming but not starting?
Updated August 2026
An outdoor AC condenser that hums or buzzes but will not kick on is most often a failed start or run capacitor, the part that delivers the jolt the compressor and fan need to break into motion. Voltage reaches the unit, so it hums, but without the capacitor's boost the compressor just sits and buzzes, sometimes tripping its internal overload after a minute. A capacitor is a $15 to $60 part; a seized compressor can total the unit. Kill the power and diagnose in the order below.
Safety first: before you open the access panel, shut off the outdoor disconnect box beside the unit and the breaker inside; the thermostat alone does not de-energize it. The capacitor stores a substantial charge even with all power off and can deliver a dangerous shock, so discharge it across its terminals with an insulated screwdriver or leave it to a pro. Never reach in with power on, and if the compressor hums, trips, and retries over and over, stop resetting it, because a compressor drawing locked-rotor current overheats with every attempt.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Failed run or start capacitor | Most common | With power off, discharge and test the dual-run capacitor, then replace one that reads out of spec or looks bulged. | $15–60 part, $150–350 installed |
| Stuck or burned contactor | Common | The relay that feeds the compressor is pitted or welded, so power arrives erratically. A tech replaces the contactor. | $150–300 |
| Hard-starting compressor | Occasional | An aging compressor needs extra torque to start. A hard-start kit gives it the boost and buys a season or two. | $50–150 kit |
| Tripped compressor overload | Occasional | Heat or high amps trip the internal overload; it may restart when cool, but a recurring trip signals a deeper fault. | Diagnostic $100–200 |
| Seized compressor | Rare | The compressor has mechanically locked and only hums. Replacement is costly and often means a new outdoor unit. | $1,500–2,800 or replace |
| Low voltage or loose wiring | Rare | A voltage drop or a loose lug starves the compressor at startup. An electrician or tech tightens and tests it. | $100–300 |
Check these first
- Set the thermostat to cool and listen at the outdoor unit. A hum with a still fan and compressor usually means a capacitor, while total silence points to power or the contactor.
- Shut off the disconnect and breaker, then open the side panel and inspect the dual-run capacitor for a bulged top, leaked oil, or a split case.
- With power off, push the fan blade by hand. If it spins freely, the motor and bearings are fine and the capacitor is the likely fault.
- Look at the contactor for burnt, pitted, or welded contacts and any melted insulation on the wires feeding it.
- Check that the breaker and the outdoor disconnect are fully on and the disconnect's pull-block is seated correctly.
Prevent it next time
Capacitors and contactors die of heat, and a dirty condenser runs hot, so coil cleaning is the real preventive habit. Rinse the outdoor coil on the recommended schedule and the compressor stays cooler, draws fewer amps, and stops cooking its own capacitor. The cost of skipping that cleaning shows up first as a $15 to $60 capacitor failure and eventually as a compressor that hums but will not start, which is a far bigger bill. A yearly rinse plus a quick capacitor check at tune-up time heads off most no-start calls.
When to call a pro
A capacitor swap is a common DIY repair if you respect the stored charge and match the microfarad and voltage ratings, and a diagnostic runs $100 to $200. Call a pro when the capacitor tests good but the compressor still only hums, when the contactor is burnt, or when the compressor trips its overload repeatedly, all signs the compressor itself is struggling. A contactor runs $150 to $300 installed, a hard-start kit $50 to $150, and a compressor replacement $1,500 to $2,800, at which point on an older unit a whole new condenser is often the smarter money. Never keep energizing a humming compressor, since each attempt overheats it.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
Can I replace an AC capacitor myself?
Many handy owners do, but the stored charge makes it the one HVAC repair that genuinely bites. Cut the disconnect and breaker, then discharge the capacitor across its terminals with an insulated screwdriver before you touch a wire. Photograph the terminal connections, match the replacement's microfarad and voltage ratings exactly, and reconnect. The part is $15 to $60. If you are not fully comfortable with the discharge step, pay a tech the $150 to $350 installed price, because a charged dual-run capacitor can hurt you.
What is a hard-start kit and will it fix my condenser?
A hard-start kit is a booster capacitor and relay that gives an aging compressor extra starting torque. If your compressor hums, struggles, and occasionally starts on a hot day, a $50 to $150 kit can coax it back to reliable starting and buy a season or two. What it will not do is revive a mechanically seized compressor or replace a dead run capacitor. Think of it as a helpful crutch for a tiring compressor, not a cure, and plan for the unit's eventual replacement.
How do I know if it's the capacitor or the compressor?
The fan test and a meter tell most of the story. With power safely off, spin the fan blade: if it turns freely and the capacitor looks bulged or reads out of spec on a multimeter, replace the capacitor and it usually starts. If a known-good capacitor is installed and the compressor still only hums and then trips its overload, the compressor itself is likely seized or shorted. A tech can confirm with an amp-clamp reading of locked-rotor current, which separates a $40 fix from a $2,000 one.
Why does my condenser hum for a bit then shut off?
That is the compressor's internal overload protector doing its job. When the compressor tries to start but cannot, it draws locked-rotor current several times normal, heats up fast, and the overload cuts it off before it burns out. Once it cools it tries again, hums, and trips once more. A dead capacitor is the usual reason it cannot start, so test that first. Repeated humming and tripping is hard on the compressor, so stop the cycle and fix the cause rather than letting it retry.