The sensors have probably crapped out on you.
Truly, they are not very reliable anyway and a little "stuff" stuck to them causes all kinds of false readings. Run plenty of water in the tank, at least 1/4 full, some Dawn detergent, about a cup will do and drive the rig around with stops, etc. The Dawn will help break down the "stuff" and the sloshing will help reduce it to liquid so it will clear out. I use one of the rinse wands with the spinning head and it works good. Anytime you are using the black tank be sure an use enough water to keep it from forming a big %#$@ in there.