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.

  • Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Oct 15 at 10:56
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    It's worth mentioning that car lights are almost always owner replaceable with very basic (or sometimes no) tools. Doing it yourself is a money saver. – GdD Oct 15 at 11:47
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    Not necessarily true. Or - theoretically. Try replacing headlights on a Smart Coupe - you wil lbe surprised how ***** complex that thingy is. Without training you end up taking the front apart. – TomTom Oct 15 at 11:59
  • Agree with @TomTom. On VWs from 2003-2011 (my experience) you can do the whole thing w/o tools and it can take an hour to get the stupid bulb to lock in. Or, you can jack it up, take the tire off, take the fender liner off then replace the bulb in about a minute, replace the fender liner and replace the tire. I can do the extra work in about 30 minutes at home and it's worth it for the lower stress level and ease of each step, even though it's many more steps. – FreeMan Oct 15 at 12:13
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    @GdD: these days there are plenty of cars where replacing the headlights is a major ordeal, unfortunately. My Mazda 6 required removing the whole front bumper to be able to replace the lights. – whatsisname Oct 15 at 14:11
up vote 34 down vote accepted

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.

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    Also, if you replace the pair, you have the good used one as a spare, in case the next failure happens at a bad time. (Keep it in the car, not somewhere else, of course!) – alephzero Oct 15 at 11:25
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    I'm just anal like that uh? – Federico Oct 15 at 12:33
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    @Federico en.wikipedia.org/wiki/Anal_retentiveness – Shamtam Oct 15 at 13:05
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    It should be good to point out that, especially because of the reasons Paulster2 pointed out, it's always good practice to change headlight bulbs before they fail. Every couple of years, as a preventive move, it will both provide much better light output and will practically eliminate any sudden failures (of course, a new bulb can blow any time but in actual practice, when replaced relatively regularly, sudden failures are very rare, provided you use decent quality bulbs in the first place). – Gábor Oct 15 at 14:16
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    @BillK That sounds like a decent separate question. I encourage you to post it as such. – a CVn Oct 16 at 8:28

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.

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.

  • Agreed, but always carry a couple as spares. They are designed to blow, but as with everything there may be variance and all we can do is consider probabilities. – mckenzm Oct 16 at 4:11
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    If you don't replace lightbulbs in pairs, how do you compare their service life? Also, if you don't buy them in pairs as well, they ought to have bigger variation due to variations in production process. – Dmitry Grigoryev Oct 16 at 8:47

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