My 2008 MK3 offside headlight unit has condensation inside. I don’t understand how or why because it is /was a sealed unit. I can see no damage from underneath or outside. I wonder whether this is a common fault or if mine is a one-off?
This happens eventually on practically every “sealed” headlight unit ever. If you’ve ever changed a bulb on most cars, you can see that they’re not really that sealed, especially once that bulb holder has been in and out a few times.
When the lights are on, it heats up inside the unit, expanding the air. Eventually, this breaks the seal (or something else does like UV, vibration or someone being too rough with the bulb holder), then when the lights are off and it cools down, the air contracts and pull a load of nice moisture-rich ambient air in, which then deposits its moisture on the nice cool surface of the lenses!
The only real cure is to make sure the unit is sealed once again… But you can’t seal it with moisture in there! As a quick job, you can remove the unit and pop it in the oven on a low temp (or just on a radiator etc) until it is dry, then let it cool in the driest room you possibly can and make sure it is resealed, including the bulb holder. To go whole hog, you put the unit in an oven at 100c for about 7 minutes, then peel off the lens, remove some old sealant and do a full refurb from there. Usually not necessary unless there’s something more wrong than just moisture.
That really helpful Ron
Many thanks indeed!
Are you 100% sure it is condensation and not a cloudy lens on the outside. They do look very similar. Droplets on the inside is obviously condensation but if it’s just cloudy looking it could polish out.
Very good point!
I’ll give the orbital polisher a go for sure. Thanks.
I had the samen issue, did not know how the moisture got in but it did.
Tried everything and was almost thinking of getting a new set of headlights.
What finally worked for me was getting a light bulb out, and removing the rubber cap on the backside of the unit,
Then stuck a hose into the opening of the light bulb, and on the other side of the hose connected the vacuum cleaner. Let it run for half an hour with some heat on the headunit, ( hairdryer, we had a Dyson fan which also can blow hot air).
It helped, Hope it stays this way but haven’t driven in wet conditions since then.
Very inventive! That would save having to take the headlight out, but won’t give you access to reseal the headlight.
Gentlemen
Many thanks for each of your splendid advices which are saved for posterity
I’m not usually lucky - so imagine my surprise when the orbital polisher removed my condensation!!
I try to postpone the removal as long as I can 
you only need a tiny crack or a break in the seal. What I think happens is that on a hot day, the air expands inside the light and some is expelled through the crack, and then when it cools, perhaps from rain or water from rinsing after a warm wash, a partial vacuum occurs.
I had this on a previous car. What I did was take the light out and drill a very small hole towards the back where it wouldn’t get wet, so it could drain, thus allowing it to “breathe”. Within a couple of weeks the condensation disappeared. Saved me a lot of money on a new light.
It is not a sealed unit.
There is always a vent of some kind moulded into the rear casing .
On my NC3.75 it looks like a little tube sticking back from the top of the rear surface and has a tiny slot in its underside. There might even be a fine foam filter in the tube to keep out insects. Your NC will be similar.
This photo is of the vent on my offside headlamp assembly, taken with help of a mirror, but I’ve flipped the image back again to what you would see if looking up from down below in the wheel-arch (ie without any mirror.)
Fog lights have a vent tube and filter as well.
