Headlights: condensation, bulbs that keep blowing, and hazed lenses

Headlights are a component where the line between “that is normal” and “that needs fixing” is blurred, and because of that owners either panic for nothing or run it on to real damage.

Condensation: when it is normal and when it is not

A headlight is not sealed by design. It has filtered breather vents: the bulb heats the air, the air expands, and the pressure has to go somewhere.

Normal: a light, even mist after rain or a wash, clearing within 20–40 minutes of the lights being on.

Not normal:

• Droplets running down the inside of the lens.
• A puddle in the bottom of the reflector.
• Mist that does not clear after an hour of driving with the lights on.
• Condensation in only one of the pair — that is an asymmetry, and it means something.

Why it matters: water on the reflector destroys the coating, and the headlight dims permanently. So condensation ruins not the bulb but the most expensive part.

What to check in the abnormal case: the integrity of housing and lens, the condition of the bulb seals, whether the breathers are blocked, and whether there is a crack after an impact.

Bulbs blowing too often

If a bulb lasts less than a season, the cause is not the bulb.

High voltage. Normal with the engine running is roughly 13.8–14.6 V. Higher means the regulator is not limiting charge, and bulbs are the first to go; covered in the alternator article.

Vibration through a broken headlight housing or worn mounts — a filament hates shaking.

Touching a halogen bulb’s glass with bare fingers. The grease creates a local hot spot and the bulb dies early. Fit them with gloves or through a tissue.

Moisture in the holder — corrosion, a poor contact, arcing.

Cheap “extra bright” bulbs — they produce that extra light precisely at the cost of service life.

Hazed lenses

Polycarbonate hazes from ultraviolet, grit, and car washes. The consequence is not cosmetic: light output falls several times over and the beam cut-off blurs.

Polishing restores clarity, but without a protective lacquer it hazes again within a season.

Replacing the lens or headlight is more durable and more expensive.

Relevant to the annual test: hazed headlights and misaimed beams are a classic advisory; covered in the test preparation article.

LED bulbs in a halogen headlight

A topic where marketing outruns physics.

The optical problem: the reflector is designed for a filament at a specific point. An LED has different geometry, so the light scatters where it should not — the driver gets glare in front and dazzles oncoming traffic.

The electronics problem: bulb monitoring measures current draw. An LED draws less, so the module sees an “open circuit” and logs a code. Resistor adapters mask the symptom without making the beam correct.

The heat problem: a halogen headlight has no designed heat path for an LED driver.

What diagnostics shows

Bulb monitoring codes — per bulb: open circuit, short, or current mismatch.

Headlight levelling codes — ride height sensors and actuators; typical after a suspension height change.

System voltage — direct evidence in any case about bulbs blowing often.

Lighting module codes — on cars with a dedicated module.

In short

• Light mist that clears in half an hour is normal; droplets and puddles are not.
• Condensation destroys the reflector, not the bulb.
• A bulb lasting less than a season means voltage, vibration, or fingerprints.
• Hazed lenses cut light output several times over and fail the test.
• An LED in a halogen headlight produces a code and a wrong beam pattern.

Download the app — bulb monitoring codes and system voltage are read with an ELM327 adapter over Bluetooth SPP on Android.