Hi, I have a 2010 A6 Avant, C6 shape.

I have water in the passenger rear footwell, above the carpet, lifted the carpet and there is loads of water under too.
Also wet under the rear passenger bench seat when lifted.

Lifted front passenger carpet and its under there too.
One more fault which is intermittent and might be related - tank system malfunction on dash, the gauge doesnt work when this fault is displayed. Suspect water is touching some wiring hence causing this.

Any help appreciated, thanks.

 

If you have a sunroof, the source of the water is most likely due to the drains being blocked. Check the drains on bother sides of the sunroof.

 

Sorry forgot to add, no sunroof.
When driving, can hear water gushing back and forth when braking or accelerating

 

Then check the drains in the plenum.

 

had same issue my sun roof drains were clogged , air compressor on low pressure blow it out the exit drains are on side of door jams. remember low air pressure.

 

Hi there, thanks for raising this. Your issue is exactly as I've come to diagnose as well. Would be keen to know how you navigated this and what the root cause was please. Thanks!

 

So I have just gone through the same issue -

You need to lift the carpet on the rear passenger side, you will need to remove the rear bench seat, but you should get away without removing the front passenger seat.

In the middle of the floor pan (front to back) there are 2 grommets (32mm) - lift the one nearest the rear seat and I bet it is full of mud and crud - directly below this grommet is a drain plug with a TINY drain hole - I have since removed this drain plug and replaced the grommet with a new grommet - the old was hard and brittle.

I have yet to determine what this drain channel is for - and why it runs from front to back - and more importantly why it carries so much water!

I did a LOT of research on this and it appears to be a common problem - the concensus of opinion seems to be that there was a manufacturing fault/issue with the front bulkhead to engine bay sealing -

My plenum chamber was missing both drain plugs - obviously been removed by previous owner - maybe he thought this was the cause of the water ingress - I poured a bucket of water into the plenum chamber ( with no drain plugs in) and a large amount of the water came out of the rear floor pan. There is NO WAY that this drain channel ( sandwiched into the floor pan of the car) and the tiny drain plug was designed to handle this amount of water.

I have not had any ingress of water since I deleted the drain plug and replaced the grommets.