When your electrical circuits have issues, you need to a troubleshoot with the right tools, likea multimeter that can test high and low voltage, resistance, continuity, current, and even frequency and temperature as this Tesmen multimeter has. Link to the tool used in the video, I do not receive a commission:
https://amzn.to/41paOyf
0:00 Intro to multi-meters
01:27 Testing high voltage
03:10 Testing low voltage
05:04 Non contact voltage test
06:19 Christmas lights
08:00 Light sockets & switches
13:53 Fuses/continuity
17:20 DC voltage
21:27 Battery test
23:40 Capacitors