Capacitor
The small battery-like part that gives your AC's motors the jolt to start and keep running.
A capacitor stores and releases an electrical charge to start the compressor and fan motors in your air conditioner and to keep them spinning. It's one of the most common AC failures — and one of the most affordable to fix. The tell-tale sign is an outdoor unit that hums or clicks but whose fan won't spin, or a fan you can nudge into motion with a stick (never do this with power on). Capacitors degrade faster in extreme heat, which is why they so often fail on the hottest day of the year. Replacing one is a quick, inexpensive pro job, not a DIY task — the capacitor can hold a dangerous charge even with the power off.