Why is there water mist in the headlights?

[NaomiFitz]

When the outside air is humid, it is sucked into the headlights, and the hot air is expelled. When the lights are turned off, the temperature inside the headlights drops, causing the humid air to condense into water mist, which adheres to the headlights. This is the reason for the fog inside the headlights. Below is relevant information: Temperature difference causes heavy fogging: The headlight assembly's rear cover is equipped with a vent pipe, through which thermally expanded gas is expelled. However, sometimes some moisture enters the vent pipe, resulting in slight fogging. This fogging phenomenon usually occurs in winter, rainy seasons, or foggy weather. Car washing or wading can cause fogging: Besides temperature differences, fogging inside the headlights is more commonly caused by improper car washing or wading. When cleaning the engine, use a wet towel instead of a high-pressure water gun. Additionally, frequent wading can easily cause fogging in the headlights. If the water depth exceeds the wheel level, water can enter the headlights through the vent pipe. Water inside the headlights not only affects illumination but may also damage the headlights.

[StBrynlee]

I've encountered the issue of moisture inside car headlights several times. The main cause is condensation due to temperature differences - when headlights heat up during night driving and cool down too quickly after parking, water droplets form on the inner surface of the lens. This happens most frequently in humid southern regions or after rain. In most cases, the moisture will disappear on its own after one or two hours of sunlight exposure, so there's no need to worry too much. However, if the droplets accumulate into water streams or even submerge the bulb base, there's definitely a sealing problem. Possible causes include aging rubber seals or cracks in the lens cover. I recommend first opening the rear cover to ventilate and dry it out, then checking the waterproof gasket. If left unaddressed, a short circuit could occur, and you'd end up paying much more to replace the entire headlight assembly.

[McDante]

From my experience, there are two scenarios when it comes to foggy headlights. The normal one occurs when the internal temperature rises after turning on the lights, then condenses into small droplets upon cooling after turning off, usually dissipating within minutes. However, if there are water marks along the edges or the fog persists for over a week, it's definitely a sealing issue. This could be due to improperly installed seals during the last bulb replacement or damage to the headlight housing seams from high-pressure car wash sprays. A DIY fix involves disassembling the housing, sun-drying it for half a day, and replacing the desiccant pack upon reassembly. For budget cars, replacing with aftermarket housings is often more cost-effective than repairs, as aged OEM seals are hard to fully restore.

[CalvinLynn]

Car light fogging is essentially a physical phenomenon, similar to window condensation in winter. When the temperature difference between the inside and outside of the light cover exceeds 10 degrees, water will condense, especially in models with high-heat halogen lights. Ventilation holes are designed to balance air pressure, but over time, these holes can get clogged with mud, leading to water accumulation. I make it a habit to check the car lights before winter each year: use a flashlight to inspect the inner wall of the light cover for cracks and check if the rubber seals at the joints of the light housing have become hard and brittle. Dealing with it is not difficult—just buy a can of electronic equipment moisture absorber and keep it in the trunk. If heavy fogging is noticed, place two packets inside to absorb moisture.