Clean using a soft brush (baby toothbrush) and check/clean the ring for dirt/crud and carefully run a cocktail stick round the ridge to clean out any muck that can give false readings, otherwise it can be a failed sensor or water ingress to the loom for the sensors ... its a well known XC90 issue.
I had mine replace under warranty , first the sensors, then the loom, after the updated loom, never had the problem again.
Hope it helps
