Everyone is familiar with a light bulb burning out. In a household fixture or a lamp, it's often accompanied by flickering, sometimes buzzing, or a loud pop when the filament overheats and breaks.
In a vehicle, a headlight burning out is often a more subtle event. You may not even notice it if you primarily drive during the day, but a headlight or taillight might burn out and stay dead for weeks before you realize it's gone. Often, it takes driving at night to notice it or even being pulled over for having a taillight out.
A typical halogen light bulb, the kind used in most vehicles for headlights and taillights as well as indicators and hazards, is meant to last around five years.
So why is it, in an older vehicle, that your bulbs burn out faster? Halogens that last five years can last even longer, but once you've had to replace them once, they may burn out every one or two years, or even more frequently depending on the quality of the bulb. In extreme cases, we've even known people whose headlights burned out every six months until they fixed the problem causing it. What is going on here?
As it turns out, there are five main reasons why halogen bulbs burn out faster than they should in a vehicle.
When your vehicle is assembled at the factory, it is generally put together in a climate-controlled environment, with many parts of the vehicle sealed against environmental intrusion. Some of those parts separate over time or are separated when you have to work on the vehicle.
One of those parts that can loosen over time is the headlight housing. The headlight housing is generally well put-together, with very little means of access without disassembling part of the housing, though this depends on the make and model of your vehicle. If you've pulled apart your headlight yourself and forgot to seal it back up properly, this could be your issue.
One reason why your headlight housing is sealed is to prevent moisture from getting in. Water is extremely adept at destroying anything it touches, from rusting metals to wearing away rocks to freezing and cracking concrete. Over time, water will erode or destroy anything.
Water also doesn't play nice with electricity, and your headlights need electricity to work. Moisture getting inside the headlight housing can do all sorts of damage, including corroding metal elements and short-circuiting the electrical contacts that power the bulb. As you'll see in one of the later reasons, short-circuits can be extremely damaging to the bulb.
Unfortunately, once you crack the seal on your headlights, it can be difficult to seal it back up. This is part of why your initial headlight bulbs last for five or more years, but even the best replacements tend to only last for three or four.
One tell-tale sign that you have moisture in your headlight enclosures is fogginess in the housing. This is condensation, typically caused by the heat of the bulb cooling off and allowing moisture in the air to condense in the housing. You'll notice this most commonly after longer drives using your headlights, which means you may miss it if you typically drive without headlights during the day and rarely drive at night.
The second most common cause of a halogen bulb popping is touching the bulb with your fingers when you install it. Usually when you're following any kind of maintenance guide or tutorial for replacing a headlight bulb – such as this one – you will see a line in there that says "be careful not to touch the new bulb" or "don't touch the glass of the bulb" or "wear gloves when handling your new bulbs".
The reason for this is the natural oils on your hands, produced by your skin. These trace oils are left on basically everything you touch. It's why you leave fingerprints, it's why you smudge glasses, and it's why things you touch over time wear down.
Here's what happens. You touch the new bulb with your fingers when you screw the bulb into your headlight enclosure. Touching the bulb leaves trace amounts of oil on the glass of the bulb. When you then fire up your car and turn on your headlights, the bulb heats up. This head is usually spread evenly around the bulb's glass, but since you touched it, there's oil in some spots. This oil heats up faster and causes uneven thermal expansion.
If you remember basic science classes, you should remember that in nearly all substances, heat causes them to expand. Heat causing your bulbs to expand is fine if those bulbs are designed properly. If they aren't, heat can damage them. Or, as in this case, the heat is trapped and spread unevenly around the bulb by the oils you left there. This uneven thermal expansion causes the glass to break, and once the glass breaks, the filament is toast.
This can also happen if you leave other kinds of residue on the bulb. One of the biggest offenders is methylated spirits, aka denatured alcohol, typically used as a solvent or fuel. Anything that leaves a residue on the glass that doesn't evaporate away will cause the same issue. These halogen bulbs are surprisingly sensitive.
You can try to clean your bulbs after installing them, but this is typically quite difficult without leaving some residue on the glass. The best option is simply to use sterile gloves to install the bulb or install it only by touching the base and avoiding touching any part of the glass.
Heat is the enemy of virtually all technology. Computers have heat sinks and fans, your car has a radiator, and even modern LED light bulbs now require heat sinks (and occasionally fans) to help keep them cool. Heat causes thermal stress. Thermal stress in a light bulb can mean that your bulb will have a reduced life span, which can cause the bulb's glass to crack, the electronics to break, or the components to fry themselves.
A halogen bulb is simply a filament inside a glass bulb, enclosed against exposure to anything that could burn it out. This filament needs to get hot to emit light. If it gets too hot, the filament can melt and break, burning out the bulb.
Heat can also burn out bulbs in other ways. As mentioned above, it can cause the glass to break, exposing the filament to the environment and burning it out. Head can also cause thermal expansion and contraction in the housing for the bulb, for the socket, and for other components, all of which can lose connections, work their way out of sockets, or otherwise break down.
Two things cause the extreme temperatures that can cause a headlight bulb to burn out. The first is extended run time. If you're driving for a large number of hours every day, and keep your headlights on during that entire time (such as if you're in an area that mandates day-time running lights, or you keep the lights on anyways regardless of local laws), you will burn through the lifespan of the bulb more quickly.
See, a bulb is not rated for "years" of life, it's rated for hours of use. A typical halogen bulb is usually rated for 500-1,000 hours. If you have a one-hour commute twice a day, five days a week, plus an additional two hours of driving scattered throughout the week, that's 12 hours per week, which is over 600 hours in a year. That's already the low-end life span of a halogen bulb in well under the total estimated life span of a headlight.
The other cause of thermal issues is weather extremes. Winter cold can cause things like your bulb housing to contract, exposing it to moisture, or separating it from electronics. The heat generated by the bulb can then heat the enclosure, possibly too rapidly, causing it to burn out. Extreme summer temperatures can also cause thermal fatigue, as the heat of the environment compounds the heat of the bulb.
Your bulb is not functioning in isolation. It's part of a complex electrical system wired throughout your car, tied into everything from your battery to your dashboard electronics to the central computer that runs everything properly.
One of the leading causes of rapid failure in halogen bulbs is voltage swings. A bulb is best designed to have a constant supply of voltage, with no spikes or dips in the electricity flowing through it. Spikes in the voltage reaching your bulb can cause the filament to take excess stress, which will eventually damage it and can risk it melting and burning out.
Voltage spikes can be caused by a variety of problems, ranging from a failing alternator to a poor-quality batter to bad fuses. Fuses are a common cause for headlight failure, in fact, and sometimes when you have frequent headlight failures it's actually because a bad fuse is sending inconsistent voltage. It might also be the fuse itself burning out, not the headlight, which itself can be caused by a short elsewhere in the electrical system.
The first step to diagnosing this issue is attaching a voltmeter to your headlight assembly, to measure the voltage coming to the bulb. If the voltage spikes frequently, this is likely causing undue stress on your bulbs. From there, you will have to trace through the electrical system in your vehicle to determine what part of it is causing the spikes.
Unfortunately, unless you're intimately familiar with vehicle electronics and have all the tools necessary to disassemble and get at the wiring in your car, this isn't something you can easily do at home. Nearly anything connected to the electrical system can be causing feedback, uneven power draw, or shorts, and it can be very difficult to identify what, specifically, is causing the problem.
Up above, we mentioned that the typical halogen light bulb is rated for 500 to 1,000 hours of operation. Keep in mind, this is for a high-quality halogen bulb, not necessarily for your average halogen bulb. It's entirely possible that the bulbs you've obtained for your vehicle are not high quality, and may not have even that level of longevity. Replacement bulbs purchased from your local auto parts store are often lower quality than what your dealer provides, and you never know what quality the bulbs are that your mechanic uses when they fix your vehicle.
Different kinds of bulbs have different life spans. Basic HID bulbs can last up to 2,000 hours, while Xenon HID bulbs can last up to 10,000. All of these pale in comparison to LED bulbs, which, when kept in the right environment, can last for 30,000-50,000 hours.
Of course, we're talking in a headlight environment. One of the longest-living light bulbs in the world is an incandescent bulb hanging in a fire station near San Jose, which has been burning for 120 years.
A well-kept LED, meanwhile, may be functionally immortal in the right situation. Neither of those situations is "in a moving, temperature-exposed, vibrating vehicle headlight enclosure."
If you're having problems with halogen bulbs burning out frequently, one of two things can happen. The first is that you diagnose what is causing the issue (moisture, temperature, voltage spikes, improper installation) and you address it. Once addressed, your halogens should have a more normal lifespan. If they continue to burn out, you'll need to do a deeper investigation to discover why.
The alternative is to switch to a more resilient bulb. LED bulbs have a dramatically longer lifespan than halogens, and you don't have to concern yourself with touching them wrong. Installing an LED swap kit can be a little more intensive than simply swapping a bulb (you may need to replace the reflector bowl, and you may need to add to or replace some of the electronics in your vehicle) but it's guaranteed to have much greater longevity than just another halogen replacement.
The exact process for swapping to an LED will vary depending on the make, model, and year of your vehicle, as well as the trim level for some models. We highly recommend that you contact a professional for advice on how to fix your headlight issues, and whether or not an LED light swap is right for you.