Try this in whatever ILS cataloging module you can get your paws on:
Search for all the records with 856 fields either containing NO $u or an empty $u.
In Millennium create lists, the search was:
856 not equal to {nothing} AND 856|u equal to {nothing}
I did that search out of morbid curiosity and got a surprise: we had 250+ miscoded and/or obsolete 856s that had piled up over time. The result was catalog records that said: ONLINE ACCESS! And then showed the user no link or a non-functioning link (as in it just reloaded the catalog record over and over again).
These are now cleaned up in our catalog. Hurrah!