A few years ago there were long posts on how to rejuvenate
the float style senders.  Search the archives for the last
10 years.  Pray that you have the post style senders....

If you do have the posts poking into the tank, you can clean
them up--and then when they still don't quite work correctly you
can add a tablespoon of salt to the tank when you fill it
and have a perfectly working system.++  See David Bierel's
web page for a more technical fix to the electronics of the display
panel.  ;-)

Richard

++ or maybe you have water you don't really like to drink
and your sensors work fine.  Our local water is from a glacier
and probably purer than what you pay $2 per bottle for; the salt
fixes that.