And in the video，you may seen like the GEN BUS OFF light is lit up，But no，in fact is the Yellow “BUS OFF”light and Tran bus off light are lighting，making it look like the“GEN BUS OFF” blue light is on.You can see in this video.The 737CL（-400）Cold and dark start，after begin start the APU，you have a clear view too see the GEN BUS OFF blue light.
https://youtu.be/J7qlWIFmXQ8