Apparently, the capacitors in household AC condensers have a limited lifespan. They are also susceptible to failure during heatwaves. When the condenser capacitor fails, there is insufficient electrical power to kick-start the compressor and condenser fan. The contactor will click, but the fan and compressor sounds will be replaced with a constant electrical hum.
See this great video for more information about why these capacitors fail: https://youtu.be/1_VtPzWxpsA?si=iEnHuE-juLnTxYyY.
Replacing the capacitor is a fairly straightforward process. Of course, one begins by disconnecting any electrical circuits that could be sending power to the AC. You should also disconnect power at the AC external contactor box. You can then remove the electrical access panel. Next, discharge the capacitor by shorting the leads with an insulated screw driver or other metal tool. At this point, it is safe to remove the old capacitor.
For a visual guide of the disassembly, check out this excellent video: https://youtu.be/9ZGfvC-PMWU?si=iJDP0hqRHo8pnNtP.
A compatible replacement must match the old capacitor in the following characteristics.
NOTE: Writing on your capacitor may be illegible due to wear. You can typically get the required info off the stickers on the fan and compressor. Check out this video for more info: https://youtu.be/ROH7CQNgwKw?si=H0hpMgYEQtxXMrSl.
Single run usually have only two leads. Dual run can have two or more.
This is is the capacitance value. Dual run capacitors are basically double-units. They will have two separate ratings, one each for the fan and compressor.
Capacitors for this application typically come in one of two voltages: 370 V or 440 V. 440 V is an acceptable replacement for 370 V, but not vice versa.
The capacitor may not be the issue. Check out this video for a rather exhaustive troubleshooting procedure for AC problems: https://youtu.be/wzAA4dlp_70?si=WO3Q-ihSdxp8ZMwk.