What are the reasons for headlight bulbs not lighting up?

[OJuliette]

Headlights not lighting up with intact fuses can be caused by loose wire connections, poor contact, short circuits, grounding issues, short circuits, and excessively high voltage regulator adjustments. Power supply failure: If the fault occurs between the power switch and the power line, the grounding test spark method can be used to sequentially check the lamp switch terminals and related wires for any short circuits. Poor contact at the bulb base: If the test spark at the power terminal of the lamp switch is good, the bulb should be removed to check if the filament is broken. If the bulb is confirmed to be in good condition, then check whether the copper piece on the bulb socket is bent or deformed, causing poor contact with the bulb base. Adjust the copper piece to ensure proper contact.