A compressor that hums or clicks but does not start is usually missing the electrical boost that gets it turning. The most common causes are a weak start or run capacitor, worn contactor contacts, or low voltage. Switch it off and call a technician, because repeated start attempts overheat the compressor.
You walk out to the condensing unit and hear a low hum, or a click and then silence. The condenser fans may spin, but the compressor never settles into its steady run. This is one of the most common calls on a warm walk-in, and it is often cheaper to fix than people fear.
A compressor that cannot start draws a large current and heats fast. Every extra attempt adds heat to the windings. Turn the unit off at the disconnect, do not keep resetting the breaker, and do not open the electrical section. Capacitors can hold a charge after power is off.
Why a compressor hums without starting
Starting a compressor takes far more current than running one. A start capacitor gives the motor a short jolt, a run capacitor keeps it efficient once turning, and a contactor connects the power. If any of those weaken, the motor tries to turn, cannot, and hums. Mechanical trouble can do the same thing: a seized compressor also hums against a locked rotor.
| Cause | Clues | Typical fix |
|---|---|---|
| Weak or failed start or run capacitor | Hum with no start. Bulged or leaking capacitor. Fan motors may also be slow. | Replace the capacitor |
| Worn contactor | Chatter, pitted contacts, intermittent starts | Replace the contactor |
| Low voltage or a lost phase | Hum and trips, other equipment struggles, utility problem after a storm | Electrician or utility check |
| Start relay or component failure | Clicks after a delay, no start | Replace the relay |
| Seized or damaged compressor | Hum, then thermal overload trips, capacitor and contactor test fine | Compressor repair or replacement |
| Pressure not equalized after a power blip | Hum right after an outage, starts after several minutes | Wait, then watch. Call if it repeats. |
What you can safely check
- Is the disconnect on and the breaker in the on position? Do not reset a tripped breaker more than once.
- Do the condenser fans spin? If they do not, the trouble may be a shared electrical part.
- Did the power flicker or go out recently? A compressor may need a few minutes to equalize before it can start.
- Is there a burning smell, scorched wiring or a visibly swollen capacitor? If so, leave it and tell the technician.
What a technician does
- Checks the supply. Measures voltage at the unit, with and without the compressor calling.
- Tests the capacitors. Reads capacitance against the rating on the part.
- Inspects the contactor. Looks for pitting and measures voltage drop across the contacts.
- Tests the compressor. Measures winding resistance and checks for a short to ground.
- Confirms pressures. A start under high head pressure can fail even with good parts.
- Repairs, then tests a full start. Watches the current draw and the pull down.
If the compressor tests good, the repair is small. If it has failed, the technician should explain what failed and why before you approve replacement. The compressor and refrigerant page covers that decision.
What to tell them
Say what you hear (hum, click, silence), whether the fans spin, how long it has been happening, and whether a storm or outage came first. In Hattiesburg's heat, add whether it worked in the morning and failed in the afternoon, because that points toward heat-sensitive electrical parts.
If the outdoor unit is silent instead, or the box is warm with the fans running, read walk-in cooler not cooling. For the parts involved, see parts and components, and for the wider set of checks, the troubleshooting guide.
Questions about a humming compressor
Can I tap the compressor or push a fan to start it?
No. Reaching into the unit is unsafe, and forcing a start can damage the compressor. Leave that to a technician.
Is the compressor dead if it only hums?
Not necessarily. Many humming compressors are saved by a new capacitor or contactor. A technician tests before condemning it.
Why does it only fail on hot afternoons?
Heat raises pressure and weakens marginal electrical parts. A capacitor that starts the compressor at 8 a.m. may not manage at 3 p.m. Cleaning the condenser lowers the starting load.