If it gets clogged enough it will set off an error code / check engine light and can cause rough idling. Some have even claimed that it can cause engine failure, but that is questionable.

If the nut becomes loose, it will cause a diesel exhaust or clicking sound from underneath the hood.

After giving mine a thorough cleaning I felt as though my VX ran and idled a bit smoother. The improvement was minor, but worth the 10 minutes it took.