The capacitor: the most common AC repair in DFW, and why it is not a DIY part

A capacitor stores electrical charge and releases it in a burst. Your outdoor unit has a dual run capacitor that serves both the compressor and the condenser fan motor. Without it the motors either will not start or draw far more current than they should, which is why a weak capacitor often shows up first as a higher electric bill and a louder start, then as a unit that hums and does nothing.

Capacitors are rated for a temperature range and a number of start cycles. A condenser sitting on the west side of a house in Plano in August runs at the top of that range for twelve hours a day. Add power flickers from summer storms and a unit that short cycles because the thermostat is set to 68, and most capacitors here do not reach the ten years the equipment is designed for. Five to seven years is typical.

Capacitors rarely fail all at once. They weaken.

• A loud hum or click from the outdoor unit when it tries to start
• The fan on top turns slowly, or starts only after a delay
• The system runs longer and the house is a degree or two warmer than the setpoint
• The breaker for the condenser trips on hot afternoons
• The unit hums with the fan still. This is the end of the line

The internet is full of videos of people swapping capacitors, and the part is cheap. Three things the videos leave out. First, the capacitor can hold a charge after the disconnect is pulled; discharging it safely is a learned skill. Second, the rating has to match: a 45/5 microfarad at 440 volts is not interchangeable with a 40/5 at 370, and the wrong one shortens the motor's life. Third, a capacitor that failed is often a capacitor that was killed by something else, like a contactor with pitted contacts or a fan motor drawing high amps, and a technician checks those in the same visit.

Typical installed cost in our area is in the low hundreds, and a reputable company will show you the readings off the old part on a multimeter before replacing it.