Condenser fan runs but compressor just hums and trips.

Diagnose the capacitor:

1. Safety first: Turn off power at the disconnect and lockout/tagout. Discharge the capacitor by shorting its terminals with an insulated screwdriver (both terminals to the case if it's a dual capacitor).

1. Visual check: Look for bulging, leaking oil, or burned spots. Any of these = replace.

1. Multimeter test: Set to capacitance mode (or use a dedicated capacitor tester). Disconnect wires and test each terminal pair: - Run capacitor (single or dual): Should read within ±5% of the rated value printed on the cap (e.g., 35µF ±5 = 33.25–36.75µF). - Start capacitor: Same tolerance check. - If it reads open (OL) or way low, it's bad.

1. Voltage rating: Match or exceed the original's voltage rating (e.g., 370V or 440V). Higher voltage is fine; higher µF is not.

Replace if:

• It's out of tolerance, bulging, or leaking.
• The compressor still hums after replacement, the issue may be the compressor itself or the start relay—call a pro.

Code note: Per NEC 440.40, capacitors must be securely mounted and accessible. Use the exact mounting hardware and don't overtighten terminals.

Pro tip: Take a photo before disconnecting wires. Dual capacitors have common, fan, and herm terminals—wire them exactly the same.

If you're unsure about testing or the compressor continues to fail after replacement, it's time to call an HVAC technician—the compressor may be seized or the start components faulty.

Safety First: Kill power at the disconnect and breaker. Verify 0V at the contactor L1/L2 with a meter. Discharge the capacitor with an insulated screwdriver across terminals (or resistor) before touching anything.

Test the Capacitor:

1. Visual: Bulging top, leaking oil, or rusted case = replace.
2. Multimeter (Capacitance Mode): Disconnect wires (note positions: Common, Herm/Comp, Fan). Read C–Herm and C–Fan. Reading must be within ±6% (or ±10% max) of the µF rating printed on the side (e.g., 35/5 µF). "OL" or 0.0 µF = open/dead. * High ESR (Equivalent Series Resistance) often won't show on cheap meters; if compressor hums/trips on locked rotor amps (LRA) but capacitance tests okay, the cap is still suspect.
3. ESR Meter (Best): Tests under load. Low ESR = good.

Pick the Replacement: Match three specs exactly from the old cap label:

1. Capacitance (µF): Dual rating (e.g., 35/5 µF). Never substitute single caps unless wiring for it.
2. Voltage (VAC): 370V or 440V. 440V is preferred (runs cooler, lasts longer). Never go lower than original.
3. Physical Case: Round vs. Oval; diameter/height must fit the bracket/strap.

Code/Pro Tip: NEC 440.9 requires compressor overload protection. If the new cap checks out but compressor still hums/trips, you have a locked rotor, bad start relay (if applicable), or failed compressor windings. Check winding resistance (C-R, C-S, R-S) – R-S = C-R + C-S. Infinite = open winding. If compressor is locked/hot, stop. Call a licensed tech for recovery/replacement (EPA 608).