Dealing with a worn idler shaft
I thought I would mention a problem I recently had and solved. I was noticing my MK3S was underextruding and that the E stepper was starting to click. After doing the usual nozzle cleaning checks, I open up the extruder idler door and found that the idler gear was very hard to move by hand. I took it apart and could move the small bearings on the idler shaft easily, but when I inserted the bearing plus the shaft into the idler gear, it was almost completely immovable. Fortunately I had some 3mm steel shaft to hand and when I tried it with a fresh piece of shaft, everything moved easily. By luck, I even had a piece already cut to just the right length (about 22mm). Reassembled with this, and printing is back to normal. It's not clear to me if the original shaft is worn, or slightly bent, or whether there is some other cause. Everything looks clean - no debris or rust.
I have recently been doing a lot of printing - about 1400 of the Verkstan face shield frames - so it's put of lot of hours on the printer and this is not the only thing that has failed. It's such a simple and cheap piece that Prusa could easily include a spare in the extra parts bag.