I have two HP M4345 printers and one HP M5035 printer that have the same problem. After one or two years, the scan darkness dropped to about half. The print function, and anything else that does not go through the scanner, is fine. I had to modify the default darkness setting to 0 or as dark as it will go just to get a decent copy. If it is a light original, the copies are poor. There are no error messages or flags that I can find.
This has happened to all of these devices and two other HP M4345 printers that have since been retired. They are at different locations and environments.
Does anyone know exactly what is causing this and if there is a solution for it?