What happens
Capacitance outside the specified range reduces the motor circuit's intended phase relationship.
Typical symptoms
Motor hums, starts inconsistently or overloads; either section of a dual capacitor may fail independently.
Checks that distinguish this cause
Safely isolate power, prove it absent, discharge using the approved method and confirm no residual voltage. Label and isolate leads, then measure each section against its label tolerance.
What can look similar
A seized motor, low supply voltage or open winding can also cause humming. Appearance alone does not prove capacitance.
Repair direction
Replace only with the approved electrical specifications and correct terminals; do not hand-start a fan to diagnose it.
Verify the repair
Verify reliable motor starts, normal rotation and operation without overload.
Equipment-specific limits
Use the OEM capacitance, tolerance, voltage rating and terminal arrangement. ECM and inverter motors have different electronics; this procedure is not a universal motor test.
Practice this diagnosis
These simulated calls exercise this component or repair family. The article describes a broader diagnostic process; a call may represent a different cause within that family.
References and applicability
Original HVACSim explanations with manufacturer and instrument-maker references for further reading. These resources support the subject; their model-specific examples are not specifications for every system.
- Fluke: How to measure capacitancePower isolation, discharge and capacitance measurement.
Editorial review: 2026-09-29. Verify the current manual for the installed model.