17

A couple of weeks ago, the right headlight in my car burnt out, so I went to the garage and had it fixed. When I picked up my car, their salesnerd told me to watch my left headlight, for "once one headlight burns out, the other often follows in short order".

At first, I found that statement a little odd. But indeed, last Friday (14 days since the previous blowout, 8 days since repair), my left headlight burned out.

Was I just unlucky and the salesperson's tale is just superstition, or is this phenomenon an actual thing?

If so, what causes it? Uneven load/voltage issues?

For reference, my car is a 2013 Kia Rio UB.

Pᴀᴜʟsᴛᴇʀ2
  • 165,084
  • 32
  • 259
  • 508
Alexander Klauer
  • 281
  • 2
  • 6

3 Answers3

34

No, the salesman was spot on. If you think about it, bulbs are designed to last about the same amount of time. If they are installed in pairs, then the secondary light which matches the first will most likely be on the road to death just as the first. This holds mostly true for higher output lights, like your headlights. Marker lights, being a much lower output, will not hold true to this.

It is (IMHO) a good idea to replace headlights in pairs. Not only due to them lasting about the same amount of time, but also because as they get older, they start to dim. If you replace one headlight, there will be a distinct difference (in most cases) between the two. I guess that may not be an issue for everybody, but I'm just anal like that. I like the headlights to be even so as not to cause weird lighting effects across my field of vision at night.

Pᴀᴜʟsᴛᴇʀ2
  • 165,084
  • 32
  • 259
  • 508
7

Lightbulb failure is a random event, which is not pre-set to happen after X hours of service or before a certain date. It is provoked by stress from vibration, repeated heating up and cooling down and voltage spikes above nominal voltage.

Having said that, lightbulbs have a predictable ageing process where their filament gradually thins out via evaporation. Thus a pair of lightbulbs from the same production batch will have quite close lifetimes, and since headlights are always switched on and off together, they tend to die within a small period of time.

I replace my headlight bulbs myself, but I tend to replace them in pairs nevertheless. The small amount of service time the second lightbulb still has is not worth the hassle of having to repeat the procedure a few weeks (months at best) down the road.

Dmitry Grigoryev
  • 2,662
  • 2
  • 14
  • 29
4

In contrast to the other contributors I have never replaced both headlights at the same time. I am 64 years old and have been replacing my own lights since I had my first car. You will sometimes get a year or more of service out of the other headlight after you replace it's mate. Headlights, like any other lightbulb have a high degree of quality and durability difference even from the same manufacturer. However, if you're already paying someone to replace your headlights, I agree, it would make more sense to have them replace both. As always, your mileage may vary.

Robert Cline
  • 91
  • 1