Halogen bulbs die the way you’d expect — one day they just don’t come on, usually right when you need them most. LEDs fail differently, and it’s a failure mode most drivers never see coming: they don’t die, they fade. Slowly, over years, until one day you notice your headlights just don’t reach as far as they used to and you can’t quite say when that started.

That gradual dimming has a name in the lighting industry — lumen depreciation — and understanding it changes how you should shop for a replacement bulb and how you should think about the one you’ve already got in the car.

What “lifespan” actually means for an LED

Here’s something worth knowing before you trust any lifespan number on a box: LED lifespan isn’t measured by “when it stops working.” It’s measured by when it dims below a usable threshold — the industry standard is called L70, meaning the point where output has dropped to 70% of its original brightness. Some manufacturers quote L80 instead, a stricter 80%-brightness cutoff. A bulb rated for “30,000 hours” isn’t promising 30,000 hours of full brightness and then sudden darkness — it’s estimating roughly 30,000 hours before you’re down to about 70% of what you started with, still working, just noticeably dimmer.

That’s also why you’ll see wildly different lifespan claims across brands selling what looks like the same bulb. A cheap bulb might hit its L70 point in 3,000–5,000 hours of real driving; a well-engineered one can genuinely push past 30,000. The chip itself is rarely the deciding factor — what happens around it is.

Heat is doing almost all of the damage

LED chips are efficient — most of the input energy becomes light rather than wasted heat, which is the whole selling point over halogen. But “efficient” doesn’t mean “cool.” The heat an LED does produce is concentrated right at the chip junction, in a housing that’s smaller and more enclosed than a halogen bulb was ever designed for.

Push an LED chip’s junction temperature much past 85°C for sustained periods, and depreciation accelerates fast — some rough industry estimates put every 10°C of sustained extra heat as roughly halving the usable lifespan. That’s why a $15 bulb with a bare aluminum shell and no fan can visibly dim within a year of daily use, while a $50 bulb with a real heat pipe and a fan lasts through multiple winters without you noticing a change.

The other usual suspects, roughly in order of how often they actually cause premature dimming:

Cheap driver electronics — the circuit regulating current to the chip; an unstable driver stresses the LED every time it fluctuates
Moisture intrusion — a housing that isn’t properly sealed lets in condensation, which corrodes contacts and can fog the lens from the inside
Chip quality itself — genuinely the least common cause of the five, despite being what most marketing focuses on
How to actually tell if yours is fading

Lumen depreciation is gradual enough that you won’t notice it day to day — you adapt right along with the dimming. A few ways to actually catch it:

Photograph the same stretch of dark road — your driveway or a consistent low-light spot — every six months or so. Side-by-side photos catch a gradual dim that your eyes have already adjusted around.
Compare left and right sides directly. If one bulb was replaced more recently than the other, park facing a wall or garage door at night and look at the two beams together. A visible brightness or color mismatch usually means the older side has degraded, not that you’re imagining it.
A cheap lux meter or even a phone light-meter app, used consistently at the same distance from a wall, won’t give you a lab-grade number but will catch a meaningful drop over time — useful mainly as a “compare against last year” tool rather than an absolute measurement.
Watch for flickering under load — turn signals, AC compressor kicking on, anything that briefly taxes the electrical system. Flickering under load is often an early driver-electronics warning sign, showing up before the dimming itself becomes obvious.
What actually slows the fade

If you’re buying new, the spec that matters more than raw lumens is the cooling hardware — an actual heat pipe or a small fan beats “aluminum heat sink” claims on a box with no fan and a thin shell. If you’re trying to protect what you’ve already got, keep the housing’s vents (if it has any) clear of dirt, and if you notice moisture fogging inside the lens, get it resealed or replaced before it starts corroding the driver board — waiting on that turns a cheap fix into a full bulb replacement.