I've got a motorcycle that has a single H4 bulb in the headlight. I know that this is a dual filament bulb, and I've been trying to find out what happens when the bulb fails.

  1. Will I have no light at all from the low and main beam?
  2. Will I have light from only the low beam?
  3. Will I have light from only the main beam?
  4. Will I have light from only the main beam but this is half as bright because only one of the filaments is working?

I've only ever had cars and bikes with the H7 single filament bulbs so this is new territory for me.

  • Welcome to Motor Vehicle Maintenance & Repair! Nov 19, 2017 at 11:42
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2 Answers 2


If you look at a H4 bulb, it has two filaments, the one next to a small metal mirror being for low beams. They share a common terminal for ground, but have individual ones for 12V.

This means each filament fails individually, without affecting the other one. Of course, when the glass breaks, both filaments fail.

And there are other failure modes not related to the bulb, but to the wiring. If the ground connection is bad, one filament might be unnoticeable darker than normal, but both together can be much darker than expected. (though both together are usually only lit when flashing high beams.)


The most common failure mode is that the dipped-beam filament breaks (the filament wears out over time by evaporation). High beam will still work (this uses only the second filament).

  • Very true, especially on motorcycles due to the headlight being on all the time while running. Nov 19, 2017 at 12:40
  • @Paulster2 in some countries the law requires ALL vehicles to have headlights on when running...
    – Solar Mike
    Nov 19, 2017 at 12:44

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