Why headlights turn yellow
The factory coating has failed and the plastic underneath has oxidized. Washing cannot touch it.
Modern headlight lenses are polycarbonate, not glass. It is tough and light, and that is what got it the job. Its weakness is ultraviolet light, which breaks it down. Manufacturers cover for that with a thin hard-coat applied at the factory.
The failure sequence
- UV exposure gradually breaks down the factory hard-coat.
- The coating begins to fail unevenly, often starting at the top of the lens, which catches the most direct sun.
- Bare polycarbonate is exposed and starts to oxidize.
- The oxidized surface becomes microscopically rough, scattering light instead of transmitting it. That scattering reads to your eye as yellow haze.
Why cleaning does nothing
There is no film on top to take off. The damage is the surface. Wash a hazed headlight and you have a clean rough surface. Getting it back means removing material until you reach undamaged plastic.
What accelerates it
Sun beats everything else, so outdoor parking in a hot, bright place is the main driver. Heat cycling off the bulbs adds to it, along with road grit at highway speed and repeated hits of strong solvent.
Most vehicles start showing it somewhere between year three and year six. Cars kept garaged routinely go twice that long.
Other questions worth answering
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