whilst cleaning the windscreen there appears to be an oily substance running down the inside of the windscreen from the rain sensor housing.

 

anyone have any ideas?

 

So it's not just me then, I've noticed this a couple of times on mine too, always the driver side of the unit,  no idea what it can be.

There's a thread on this somewhere I think. There's a clear gel pad between the windscreen and the sensor. Mine also leaks slightly in extremely hot weather if the cars not in the shade - only happened a few times.

This does occur on other marques as well.  As long as your auto-wipers still function correctly, just clean up the mess and forget about it.

On 10/22/2022 at 12:14 PM, Eric Bloodaxe said:

There's a thread on this somewhere I think. There's a clear gel pad between the windscreen and the sensor. Mine also leaks slightly in extremely hot weather if the cars not in the shade - only happened a few times.

I've seen a grease/oil run on my screen and wondered where it was coming from. It's on passenger side and obscured by the mirror, so doesn't bother me. What is this gel used for, stop vibrations and/or stick it to the screen?

2 hours ago, Jim H said:

What is this gel used for, stop vibrations and/or stick it to the screen?

That's the general idea, and according to blurb I've seen somewhere: " it smoothes out irregularities between the optical sensor and windshield glass, creating a defined optical interface". 

No doubt others will know more - I'm not a fan of auto wipers and always switch mine to variable intermittent if that option is available in the car settings menu. I know many love 'em, though!😀

It's probably something to do with the refractive index of the gel being closer to that of the glass than air so it affects the angle of view of the sensor.

4 minutes ago, mjt said:

It's probably something to do with the refractive index of the gel being closer to that of the glass than air so it affects the angle of view of the sensor.

Wow !!

That sounds like you know your stuff.