Capacitors

A weak or failed capacitor is one of the most common AC failures, and one of the cheapest fixes.

The short version

• Kill power, then discharge the capacitor safely before you touch it.
• Look for bulging tops or leaking oil. That's failed.
• Test with a meter that reads microfarads (µF). Compare to the rating printed on the capacitor.
• A compressor that hums and trips, or a fan that won't start without a push, points to the capacitor.

Numbers

• Common dual capacitors: 35/5, 40/5, 45/5, 55/5 µF at 370 or 440 V
• Tolerance: within about 6% of the label

Shop tip

Replace with the same µF rating and the same or higher voltage rating.

Try it on a service call

Put the lesson to work on a hands-on troubleshooting case.

• AC ices up and barely blows

Keep going on air conditioning

Learn it

• Rooftop AC
• Airflow and coils
• Thermostats and AC control boards
• Heat pumps and heat strips

Prove it

• AC hums and trips
• AC runs but barely cools
• AC ices up and barely blows
• AC kicks off on hot afternoons
• AC does nothing, thermostat is blank

See it in 3D

• Rooftop AC 3D wiring model

On the job

• Air conditioner
• Thermostat
• Heat pump / heat strip