AC Capacitor Problems and Replacement

What a Capacitor Does

A run capacitor provides the electrical phase shift that motors need to start turning and to run efficiently. Residential systems typically have a capacitor serving the compressor and the condenser fan — often combined into a single dual-run capacitor — and sometimes a separate one for the indoor blower.

Capacitors degrade with heat and time. West Texas summers are hard on them: an outdoor cabinet in direct sun, running long hours in high ambient temperatures, ages a capacitor faster than the same part would age in a mild climate. Heat and long run hours are among the conditions that can shorten capacitor life.

Symptoms That May Indicate Capacitor Failure

These symptoms overlap with contactor problems, motor failure, wiring faults and compressor issues, so a measured test is what confirms the diagnosis. A technician compares actual capacitance against the rating printed on the part.

• The outdoor unit hums but the fan does not spin
• The outdoor fan does not start normally even though the unit hums or attempts to run
• The compressor tries to start, clicks and shuts down repeatedly
• Warm air at the vents while the indoor blower runs normally
• A clicking sound at the outdoor unit followed by silence
• Occasional breaker trips when the system attempts to start

Important Safety Warning

Capacitors store electrical energy and can remain charged after the power is switched off. Contact with the terminals can cause serious injury. Do not reach into the condenser cabinet and do not touch fan blades, capacitors, wiring or any electrical component. Do not open the electrical compartment of the outdoor unit, and do not attempt to test a capacitor with household tools.

Never try to start, spin, push or nudge the outdoor fan blade by hand or with any object, whether the unit is running or switched off. Turn the system off at the thermostat and the disconnect, then call for qualified help.

If you see a capacitor that is visibly swollen, leaking or burned, that is information to relay when you call — not an invitation to remove it yourself.

What Replacement Involves

A technician disconnects power, safely discharges the capacitor, verifies the ratings, installs a correctly rated replacement, and then tests system operation and motor amperage. It is generally a short repair, and the part itself is inexpensive compared with most HVAC components.

If a new capacitor fails again within a short period, that is a signal to look further — a motor drawing high current, a failing contactor, high head pressure from a dirty coil or restricted airflow can all shorten capacitor life. Replacing the part repeatedly without investigating the cause is not a solution.