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Headlights burn out at same time

This thought occurred to me as my wife and I were driving down a dark highway. If headlights are installed in a new car at the same time, then theoretically, they should burn out at the same time. Do you then drive with your brights on until you get home?

While it can happen, it’s unusual for both headlights to burn out at the same time. This is not something I’d waste any time worrying about.

If that happens, check to see if you are being followed by UFOs.


They are unlikely to burn out on the same drive, but if one goes you should replace both. They are both rated for the same lifespan, and if you only change the burned one the other may be ready to go out too. Headlights are too important to safety to not do this.

The headlights have a rated average life. Let’s suppose that the average rated life is 1000 hours. This doesn’t mean that the bulbs will light for exactly 1000 hours and then burn out. There is a statistic called the variance that is calculated, and from the variance, a value called the standard error of the mean is derived. Let us suppose that the standard error of the mean for the bulbs is 50 hours. Then 2/3 of the bulbs should last between 950 hours and 1050 hours. 95% of the headlight bulbs will have a life between 900 and 1100 hours. These results are obtained by taking a sample of the headlight bulbs, noting the time that each headlight bulb lasts and then calculating the mean, variance and standard error of the life of these headight bulbs.

The same is true for household light bulbs. If you put three identical new bulbs in a light fixture that has three sockets, the bulbs don’t all burn out at the same time. If the bulbs are in a ceiling fixture, it does make sense to change all the bulbs for convenience. I was on a university campus where a crew went around campus and changed all the lightbulbs on a periodic basis. This actually saved money because 1) the bulbs were bought in huge quantities; and 2) the crew could make repairs to the fixtures (e.g. they could change the ballast coil on a flourescent fixture if needed) so that in a large building, fewer janitors were needed as the janitors were not responsible for changing the light bulbs.

In the olden days of sealed bulbs it was replace them as they burned out. I read the stories if when one goes replace the other, naturally I ignored the advice and bought one tungsten bulb and replaced it, sure I should have bought the pair at a discounted price for 2, but I did not. 3 weeks later there I was replacing the other bulb. Brights is obnoxious to other drivers, lights are a safety factor to other drivers, If you have to do it one time I forgive you, if you do it many times I curse you.

I say no to continued driving at night with no headlights, so brights it is, until you must dim.

If it happens again, have your charging system voltage checked out. A high system voltage (above specs) will shorten the life of the bulbs in the vehicle.

Are you sure they both went out at the same time? My guess is the first one went out and you did not notice. It is easier than most people thing to not notice. When the second one burned out, likely not long after the first, it appeared that both burned out together.

As I read the original post, I think that the question was a theoretical question and that the headlights hadn’t burned out. You are correct that many people don’t notice when one headlight burns out. If the bulbs are the same age, the other usually follows shortly thereafter. Both bulbs have the same rated life.
I have a running battle with one person in my church who is on our house committee. We have outdoor floodlights that come on at night and each year, several of the bulbs are burned out. I hate being up on a ladder, so when I replace one of the floodlight bulbs in a fixture, I replace both of them. This person is always on my back because she says I am wasting money replacing a good bulb. Of course, she won’t climb the ladder to replace the bulbs. I have noticed that when one flooodlight bulb goes out, the other ususally follows suit in a week or two.

Are they incandescents? If so, perhaps you can convince her that it’d be a good idea replacing all the bulbs with new long-life low-energy flourescent, quartz-halogen, or LED array bulbs (if LED arrays are available for floodlights).

I heard on talk show radio that energy-saving legslation has been passed that will ban incandescent bulbs in the next few years anyway. I’ve been stocking up.

These floodlight bulbs are incandescents. I hope to replace them low-energy flourescent. I tried them in one fixture to be certain they would start in cold weather. My other idea is to just replace them with gas mantles and go to gas lights (This goes back to the days when I used to make the 80 mile trip from Carbondale, Ill where I was in school to Gas Light Square in St. Louis back in the early 1960’s. Someone told me that Gas Light Square is all gone now).

Not that it has any real relevance, but I had a headlight burn out on Christmas Day on an old Pontiac I had, then the following year, on Christmas Day in the same spot no less, the light on the other side went out.

I have been involved in construction projects where all new fluorescent light fixtures and bulbs were used. Then 3 or 4 years later, most of the new bulbs fail within a couple of months of each other. Pretty consistent quality control with those at least.

Bulb manufacturers always tell you to replace in pairs. I NEVER do. I’ve had a bulb go…and then the other one went over a year later. Why replace a perfectly good bulb. I usually buy them in two’s. Then keep the other one around until needed.

Replacing headlights in pairs is a bit AR. Why not replace ALL the lamps in the car too? That makes no sense. I just replace them as they go bad. This give me a great excuse to hang out in my garage once in a while to avoid nasty household chores. Give me a ratchet and screwdriver and I can fix anything. Give me a hammer and saw and I create sawdust and soft tissue injuries…

I have four LED floods and they are amazing. Not best for all applications but the amount of light for the low wattage is very good.

Well if you’re going to replace lights that are in a pain-in-the-butt location, like behind the dash, it’s a pretty good idea to replace them all at once.