Very helpful

 

A little OT and borderline necrotreading, but but any ideas what the plug/lead on the cabin/pollen filter housing in step 3 (post#2) is? Never seen this before - wondering if something is missing om my A6.

 

The plastic clip in the middle is really just a clip to secure the filter. Not really needed or useful as the cabin-filter doesn't move much and the clip doesn't even touch the filter.

The plug on the right side of the picture is the air quality sensor - it controls the button that looks like an A with a arrow-circle around it.

 

Thank you. Do you happen to know; was this sensor optional or maybe a post-facelift addition? No clear indication mine ever had one ('99 Euro A6 C5).

 

From a little Google search it looks like the pre face lift version (98-2002) had this thing too but only if you have an A/C unit as it is operated by that module.
I cannot tell you if your specific model had it or not - Look around in the engine bay by the cabin filter and see if you can find it. If you have VCDS you can scan the hvac unit for codes and see if it's complaining about it - that would be a good indicator you have it but missing or bad.

 

I can't see any physical indications that there ever was a sensor like this on mine, nor any codes on the HVAC (AC was standard on all euro C5's I think). My recirc-button doesn't have any "A" on it, so I guess this might be a market specific option.