Why do headlights go cloudy or yellow — and can they be restored?

What causes cloudy headlights?

Plastic headlight lenses are light and tough, but they're protected by a factory coating that doesn't last forever. UV light from the sun, heat, road grit, harsh cleaning chemicals and years of weather gradually break that coating down.

Once it's gone, the plastic itself starts to oxidise. You see it as a hazy, frosted or yellowed surface — usually worst on the top of the lens, which gets the most sun.

Why it matters: light output and the MOT

A cloudy lens scatters and blocks light, so the beam is weaker and less well defined at night. It also makes an otherwise tidy car look older.

Headlamp condition and aim are checked at the MOT. The DVSA's inspection manual sets out what testers look for, and you can check whether your car's headlamps have been flagged at a previous test using the GOV.UK MOT history service.

Restoration vs replacement

Because the damage is on the outer surface of the lens, it can usually be treated rather than replaced. Headlight restoration removes the oxidised layer and brings back clarity, which is far less disruptive than buying new headlamp units.

Replacement is still needed if a lens is cracked, if moisture is getting inside the unit, or if the damage is inside the headlamp rather than on the surface.

• Hazy, frosted or yellow outer lens — usually restorable
• Cracked lens or condensation inside — needs repair or replacement
• Dim beam with a clear lens — likely a bulb or aim issue, not the lens

Keeping headlights clear afterwards

Regular washing with car-safe products, and avoiding harsh chemicals on the lenses, helps restored headlights stay clear for longer. Cars that live outdoors will always be exposed to more sun, so keep an eye on the lenses.