I'm close to starting a new thread as a kind of "master thread" about this generation of printers, because there's a lot of info I've gleaned over my 10+ years working with them. I've got two 9400s now, both with over 100,000 sq/ft on them (one of those was entirely by me, the other I bought for cheap with already 160K on it, but it's MUCH cleaner than my original unit-- whoever was running it must have run one paper, consistently long runs unattended).
I've noticed recently I've been having some false head replacement errors that I'm pretty sure I've narrowed down to poor electrical contact with the heads. I (just last night and this morning) took all 4 heads out of my two printers in service mode using the CR_UNLOCK and cleaned both the nozzle plates with some ink owl printhead flush, and cleaned the contacts on the back with first a white eraser and then some contact cleaner on a piece of cloth. I also used the eraser on the contacts on the inside of the head carriage-- swiping downward as all of the pins face in that direction to avoid bending them. This seems to stop it from randomly throwing head errors in a lot of cases.
The reason why I cleaned the nozzle plates is that I noticed that on both my machines the blue ink was acting strangely in head alignment checks, the center lines on a bidirectional calibration showed up doubled like they weren't jetting properly, but after cleaning manually it's much better. There must be thicker pigment in that ink, like on epsons with their orange.
It's also worth cleaning the wiper blades when you have the carriage unlocked, because they will get gunked up and then not be able to clean well anymore.
Look up the service manual for the ipf8300, there's a really fascinating section (Chapter 2) where they actually go into "theory of operation" basically explaining every element of how the printers work from a technical level from data input to nozzle output-- much like the way electronics service manuals were back in the 1970s.
