Water on floorboard back passenger side
It probably has something to do with a drain pipe that is clogged.
Sunroof drain clog or worn window weatherstripping are common causes.
We had this happen in our car. Near the front passenger side by the center counsel there is a water drain for part of the AC unit that got clogged up. Cleaned it out and this fixed the problem.
Looking for a Used S40 in your area?
CarGurus has 1,225 nationwide S40 listings starting at $1,995.
Search Volvo S40 Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Volvo S40 Experts