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Similarly to this question, the replacement tail/brake combined lights I bought from Halfords for a Chevrolet Lacetti advised that the bulbs be replaced in pairs. They're dual filament conventional bulbs.

Is there any advantage or reason to replace this type of bulbs in pairs other than bolstering the manufacturer's profits? I've only replaced the broken one so far and they look equally bright to me.

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4 Answers 4

up vote 6 down vote accepted

Each bulb have some sort of life time expectancy so if two lamps are installed at the same time and one fails after a long period of time you can deduce that the other can be close to failure as well so for security some people and manufacturers ($$$$) recommend replacing both at the same time.

The thing is that there is no rule since a lot of factors come to play when estimating the life expectancy of the bulb, just to mention a few.

Length of wiring (left and right side) some times indicator or other lights on the car have a different wire lengths.

Quality of the bulb two bulbs will never be the same even if the same quality check is applied to both, the filaments are sooooo delicate that what looks ok to the naked eye may be far from the reality.

Condition of the bulb receptor Bad contact, water, over heating, poor maintenance,etc, etc.

If one goes off buy two, replace one, and keep the other in the car just in case it is needed.

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@Rory Alsop I love the edit as it helps me to improve my grammar, Apologies for my bad English, I'm just learning. –  user983248 May 31 '12 at 11:55
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Sort of. Older filaments in headlights can be noticeably duller. If you are OCD, it might benefit you. Typically when one bulb goes out, the others that were replaced at the same time are probably near their end of life. Or, perhaps not. I don't replace things that aren't broken.

Otherwise, it's just more money for the parts store.

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I'd rather have them both on a different replacement schedule. After all, if one goes out, the last thing you want is for the other one to go too before you get around to replacing the first... :-) –  R.. Jun 7 '12 at 6:45
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My mom's Prius HID bulb died (under warranty) so she took it in to get it replaced. The dealer replaced one of the bulbs and it was a completely different color than the original. I finally bought her two new bulbs from Amazon and replaced them for her because it was so bothersome. In a situation like that it is definitely worth it to replace both at the same time to make sure that they are the same color and brightness.

In your situation - if both lights (new and old) are the same color and brightness there really is no reason to replace them at the same time, especially if one light broke and did not burn out on its own. If you have two headlights that are the same age and one burns out from usage then you can bet the other one is going to go sometime soon.

As a rule of thumb, I always keep an extra in my car - just in case!

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If one headlight dies and gets replaced then the new lamp is usually brighter then the old one. This bothers me and I usually replace both.

I keep the working old lamp and the next time a lamp fails I replace it with that, resulting in two slightly dimmed headlights.

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