I had an intermittent Adblue warning on an A4 Allroad recently. Then I noticed a pattern. Initially I got the 650 miles warning which dismissed itself but was left with the smaller warning above the clocks with a spanner and the word Adblue. This disappeared after 2-3 miles. So, an intermittent error message.
If you get this do check state of battery charge. Low battery caused the error on mine, hence why it disappeared after 2/3 miles of driving. Put the battery on a trickle charge for a few hours and guess what - no faults shown.
I’d hate to think that a dealer would change parts and charge the customer when all that was needed was a fully charged battery…
Colin