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That is usually a sign of the device getting old, though other factors may contribute.
How old is your Furuno Nav Net device? measured from date of purchase, and also measured as in-use time (as opposed to time in storage, etc.)
It might also be a sign of excessive salt water or salt spray exposure. And more than likely, it can have been caused by being in the sun on hot still days, i.e. overheating. Is exposure to maritime conditions or salt water or spray likely to have happened? Has it been used in a hot southern climate at any time or frequently
(as opposed to in a climate north of roughly Delaware (on the East Coast of the US) or somewhere north of San Francisco (on the West Coast) or perhaps north of roughly the latitude of Chicago in an interior or Mississippi/tributaries context?)
Well, at minimum the display part of it, the 'panel', will have to be replaced in a shop. Probably not something you can do yourself.
As for whether it should be repaired, or replaced, that depends on a simple question of economics (as far as I am concerned, and being in ignorance of any other facts or preferences of yours that might enter into the decision...):
- how much does it cost to have the panel replaced, as opposed to replacing it?
This can be determined by contacting a shop that could service this problem with this device, and getting an estimate of charges for the repair from them.
Then you factor in, that the repair, if done, may not be permanent - may not last as long as a fully warranty-covered new unit would be likely to - and that something else may go wrong that isn't the screen, in a 10-years-old device (in the S. FLA climate, too... electronics don't like heat, though military-specification devices can stand it).
Let me know your local zip code, and I will give you contact information and addresses for nearby shops in your region that you might take it to for repair, and that you certainly could ask for an estimate of repair charges.