Capacitor failure occurs when the electrical storage component loses its ability to hold and release electrical charge, preventing motor startup and operation. Capacitors store energy to help start the compressor and fan motors; when they fail, motors cannot develop sufficient torque to overcome inertia. Failed capacitors are the most common HVAC failure, accounting for approximately 30 percent of service calls.
Types and Symptoms
Run capacitors (35 to 100 microfarads) maintain continuous motor operation, while start capacitors provide momentary boost during startup. Symptoms include humming sounds without motor rotation, reduced airflow, delayed compressor engagement, and complete system shutdown. Capacitors typically fail after 3 to 5 years or 40,000 to 80,000 cycles due to heat stress and electrical surges.
Testing and Replacement
Technicians use multimeters set to capacitance mode to test microfarad (μF) ratings against nameplate specifications. Replacement requires exact matching of voltage rating (370 or 440 volts) and microfarad value. Capacitors cost $20 to $80 each and should be replaced as matched pairs to prevent uneven load distribution.
Prevention
Installing surge protectors and maintaining proper ventilation around outdoor units reduces heat exposure and extends capacitor life.