Your heat pump or AC outdoor unit is humming, the fan is not spinning, and the house is getting cold. The first fear is a dead compressor, the most expensive part in the system. In our experience across Tacoma and Pierce County, it is far more often a capacitor, a part about the size of a soda can. This guide explains what each part does, how a technician tells them apart with real readings, and why ductless inverter systems do not follow the same rules.
What the capacitor and compressor actually do
The compressor is the pump that moves refrigerant through the system. On a conventional single stage or two stage outdoor unit, it is a motor that needs an electrical boost to start and stay efficient. That boost comes from the run capacitor, which usually also serves the outdoor fan motor in a single dual capacitor.
When the capacitor weakens, the motor gets power but not enough starting torque. It sits still and hums, drawing high current until an internal overload trips. To your ear, a stalled compressor and a weak capacitor sound almost the same.
Why capacitors fail in the South Sound
Heat and long run times
Capacitors are rated for a temperature limit. On a hot afternoon a dark outdoor unit against a south facing wall can bake well above air temperature. Heat waves like the one in June 2021 produced a wave of capacitor failures across the region, and many more weakened without failing outright.
Winter cycling and defrost
Heat pumps here run most of the year and go through defrost cycles on damp mornings near freezing. More starts mean more stress on the capacitor. A unit that started fine in October may grunt and hesitate by January.
Salt air and moisture
Near the water, from Ruston Way to Gig Harbor, salt air corrodes capacitor terminals and contactor contacts. Corrosion adds resistance, which looks like a weak capacitor even when the can itself tests fine.
How a technician tells them apart
Symptoms alone do not decide it. A proper diagnosis uses three measurements before any part is ordered.
| Test | What it shows | Points to capacitor | Points to compressor |
|---|---|---|---|
| Capacitance (microfarads) vs the rating on the can | Whether the capacitor still stores enough charge | Reading more than about 6 percent below rating | Reading within rating |
| Compressor winding resistance | Whether the motor windings are intact | Windings read normal | Open or shorted winding, or a winding shorted to ground |
| Amp draw on startup and while running | How hard the motor is working | High locked rotor amps that drop to normal once the capacitor is replaced | High amps even with a known good capacitor, or tripping the overload within minutes |
A careful tech will also check the contactor, the fan bearing, and the supply voltage, since a pitted contactor or a seized fan can mimic both failures. If a new capacitor is installed and the compressor still will not start, that is when the compressor itself is in question.
What you can safely check yourself
- Look at the outdoor unit while it is calling for heat or cooling. Is the fan spinning? Is the unit humming?
- Check the breaker and the outdoor disconnect. Reset a tripped breaker once only. If it trips again, leave it off.
- Note whether the problem started after a power outage or on a very hot or very cold day.
Do not open the panel or touch the capacitor. A capacitor can hold a dangerous charge even with the power off. Our guide on when to call a tech vs DIY covers where the line is.
Why ductless inverter systems are different
Inverter driven systems, including the Mitsubishi Electric and Daikin ductless units we install, do not use a run capacitor to start the compressor. A circuit board converts power and ramps the compressor up gradually. When something goes wrong, the system usually shows an error code or flashing light pattern instead of humming. Diagnosis then means reading the code, checking sensors and board voltages, and testing the compressor windings through the inverter board. If your ductless unit is blinking, write down the pattern before calling.
Some ducted variable speed systems, including higher end American Standard heat pumps, also use inverter drives, so the same logic applies to them.
Why this matters for your wallet
A capacitor is one of the least expensive repairs on a heat pump. A compressor is one of the most expensive, and on an older system it often leads to a replacement conversation. Ask your technician which readings led to the diagnosis. If a compressor is condemned without capacitance, winding, and amp readings, get a second opinion. If your outdoor unit is also icing heavily, see our guide on why heat pumps ice up in Puget Sound weather.
Capacitor vs compressor: FAQ
Can I run my heat pump with a bad capacitor?
No. A compressor trying to start against a weak capacitor overheats its windings. Repeated stalled starts can turn a small repair into a compressor failure.
How long do capacitors last?
Many last 10 years or more, but heat, frequent starts, and salt air shorten that. It is common to see one fail in the first hot or cold snap of a season.
Does a bulging capacitor always mean it failed?
A bulged or leaking capacitor has failed. But most capacitors fail electrically while looking perfect, so a normal looking one still needs a microfarad test.
My ductless unit is blinking. Is that a capacitor?
Usually not. Inverter ductless units report faults through error codes. The code points the technician to the sensor, board, or compressor circuit to test.
Outdoor unit humming and not running?
Turn the system off at the thermostat to protect the compressor, then call us. We test before we replace, and we show you the readings. Call (253) 370-8641 or send us a message.