Hi Richard,
I get your point.
What gave me the knowledge is that I have two HP2840, printers.
One I bought about 7 years ago and is working fine, printing and scanner/copy.
For our office we needed another printer, and I came across a second-hand HP2840 unit for € 60 including a new drum and Toner Cartridge.
This second-hand printer was working fine when I bought it, installing it at our office I was confronted with the Scanner Error 5 issue….
So I started to take the unit apart and tried to find out what was causing it. Because I could not find a clear reason for it, I started swapping parts from the one (first) unit on to the other.
By accident I swapped the top casing with glass plate to the first (good working printer), at that point my first unit that had no issue before, also gave the Scanner Error 5.
That was a shock at first, thinking that I have 2 faulty units, caused by my own actions..., than I found out that I swapped the top casing. So I took it a part and placed the right top casing back and my trouble was over.
Then I placed the top casing from my first printer to the second-hand printer and the Scanner Error 5 issue was gone right away.
So I started to investigate the differences between the two top cases, there I saw that the white reference line was off by 3 mm.
I moved the white reference line as described in my frist post and the printer is working without any error ever since. Even Until this day!
This second hand printer is older than my first printer, so I think that the people at HP found out about this issue and corrected the fabrication process.
That a printer suddenly is acting funny after years of trouble free printing can come forth because of a decayed light bulb. I can very well imagine that the optical sensor (reference mark) is getting at the edge of the tolerance when the light bulb is decaying over time. That is why other forum members get good results by replacing the light bulb.
So by manually move/adjust the reference line (the white line) we can get it within the tolerance area of the scanner unit.
I hope that this makes sense for you too.
And with out making any costs on expensive parts we can all use this printer for many prints and scans to come.
Best regards Bert