What causes one headlight to be on while the other is off?

[Phil]

One headlight being on while the other is off is due to a blown fuse or a short circuit in the bulb. Here are some maintenance precautions for car headlights: 1. Replacement: Car headlights should maintain good sealing, especially to prevent contamination and damage to the reflector coating. When replacing bulbs and diffuser glass, avoid touching the lens with your fingers to prevent leaving sweat or oil marks that could cause the reflector to lose its luster and reduce reflective efficiency. 2. Regular inspection: Periodically check and adjust the headlight beam to ensure the headlight's illumination distance meets requirements and that the high and low beams are aligned. The diffuser glass should be correctly installed as specified to prevent loosening or displacement and to maintain the vertical alignment of the prism.