By Mario Vaghei, General Manager, Aire One Peel Heating & Cooling · Published November 9, 2026 · Facts verified November 5, 2026
Short answer
When an outdoor air conditioner or heat pump hums but will not start, two small electrical parts are the usual suspects. The capacitor stores a charge and gives the compressor and fan motors the boost they need to start and keep running. The contactor is the switch that turns the outdoor unit's main parts on and off when the thermostat calls. Both wear out, both sit in a high-voltage compartment, and both should be tested and replaced by a trained technician, not by the homeowner.
On this page
What a capacitor does · What a contactor does · Capacitor vs contactor at a glance · Signs to watch for · Why capacitors fail · Why this is not a DIY repair · What to do when the outdoor unit hums · When to call us · Frequently asked questions
What a capacitor does
Carrier describes an air conditioner capacitor as a part that "acts like a powerful battery for your air conditioning system". Bryant puts it this way: a capacitor "provides the electrical boost needed by certain motors and compressors during startup and operation". There are three kinds: a start capacitor, a run capacitor, and a dual run capacitor that serves the compressor and the fan motor from one part.
Heat pumps use the same part. Trane explains that the capacitor gives the compressor motor and fan motor "a little jolt of electricity" to start, then supplies a steady current to keep the unit running. If your system is a heat pump, our heat pump repair page covers what a service visit looks at.
What a contactor does
Bryant defines a contactor as "an electrically controlled switch that helps turn major components of the outdoor unit on and off". When the thermostat calls for cooling, the contactor closes and sends power to the compressor and the outdoor fan. When the call ends, it opens again. If its contacts are worn or stuck, the outdoor unit may not start, may start and stop, or may buzz.
Capacitor vs contactor at a glance
| Capacitor | Contactor | |
|---|---|---|
| What it is | A part that stores an electrical charge | An electrically controlled switch |
| Its job | Gives the compressor and fan motors the boost to start and run | Turns the outdoor unit's main parts on and off |
| Signs it is failing | Humming, a fan that is not spinning, reduced cooling, frequent cycling, higher energy use | The AC will not start, buzzing, intermittent operation |
| Typical lifespan | About 10 to 20 years | About 5 to 15 years, depending on the source |
| Who replaces it | A trained technician | A trained technician |
Table: Carrier, "What is an AC Capacitor?" and "Air Conditioner Parts Guide"; Bryant, "AC Parts". Bryant lists 5 to 10 years for a contactor and Carrier about 5 to 15. Read November 5, 2026.
Signs to watch for
- The outdoor unit hums but the fan is not spinning. Bryant lists this as a sign the capacitor may need replacing.
- The system struggles to start, or clicks every few seconds. Trane notes you may hear a failing capacitor clicking, and a loud hum from the motor when a start capacitor has failed.
- The AC will not start at all, buzzes, or runs on and off. Carrier lists these as contactor symptoms.
- Less cooling, more frequent cycling, a higher bill. Carrier lists these for a weak capacitor.
- A burning smell or a bulging, cracked or leaking part. Trane lists bulging, cracks and oozing liquid as visible signs of a failed capacitor. Switch the system off.
The symptoms overlap, and other faults cause them too. Trane's own check is a useful one: if the compressor is running and the fan is turning, the problem is something other than the capacitor.
Why capacitors fail
Trane names overheating as the biggest reason, followed by plain wear over the years, long run times such as a heat wave, and an aging compressor that draws more energy each time it starts. That last one matters: a capacitor that keeps failing can be a sign of a compressor nearing the end of its life.
Why this is not a DIY repair
Carrier warns that capacitors "store high-voltage electricity that can cause serious injury or death, even when the power is disconnected." Trane says capacitors "should only be tested, discharged, or replaced by a trained HVAC professional". Bryant adds that "a qualified technician should diagnose the underlying cause before a component is replaced", because swapping a part without finding out why it failed can leave the real problem in place.
What to do when the outdoor unit hums
- Switch the system off at the thermostat. Carrier recommends not running an air conditioner with a bad capacitor.
- Do not open the electrical panel on the outdoor unit. The capacitor can hold a charge with the power off.
- Check the simple things you can see: the thermostat setting, the furnace filter, and the breaker for the outdoor unit.
- Book a service call and describe what you heard: a hum, a click, a buzz, or nothing.
If the unit is older and the repair list is growing, our guide to repairing or replacing an air conditioner helps with the decision.
When to call us
If your outdoor unit hums, clicks or will not start, book air conditioner repair in Mississauga. For a heat pump, a yearly check of the electrical parts is part of heat pump maintenance. Call 905-564-8545.
Frequently asked questions
What is the difference between a capacitor and a contactor?
A capacitor stores an electrical charge and gives the compressor and fan motors the boost to start and run. A contactor is an electrically controlled switch that turns the outdoor unit's main parts on and off when the thermostat calls.
Why does my AC hum but not start?
A failed capacitor is a common cause: the motor hums because it has power but not the boost it needs to start. A worn contactor, a seized fan motor or a failing compressor can cause similar symptoms, so a technician should test before replacing anything.
How long does an AC capacitor last?
Carrier and Bryant both say most air conditioner capacitors last about 10 to 20 years. Heat, long run times and an aging compressor can shorten that.
How long does an AC contactor last?
Carrier's parts guide lists about 5 to 15 years and Bryant lists 5 to 10 years. Contacts wear a little every time the unit starts.
Can I replace an AC capacitor or contactor myself?
It is not recommended. Carrier warns that capacitors store high-voltage electricity that can cause serious injury or death even with the power disconnected, and Trane says they should only be tested, discharged or replaced by a trained HVAC professional.
Sources and references
- Carrier, "What is an AC Capacitor?", read November 5, 2026.
- Carrier, "Air Conditioner Parts Guide", read November 5, 2026.
- Bryant, "AC Parts", read November 5, 2026.
- Trane, "Heat Pump Capacitor", read November 5, 2026.