Getting MMU2 to work took me several months. I found it practically unusable until I upgraded the printer to MK3S. The “S” upgrade added a working filament sensor on the extruder. Before the upgrade, MMU would push a measured length of filament in hopes it would engage with extruder teeth. It does, most of the time. “Most of the time” does not cut it when a print has several hundred filament changes.
The problem with MK3S upgrade — you are supposed to upgrade MMU2 to MMU2S at the same time; MK3S extruder does not accept MMU2. It took me some asking around to find this solution:
This still does not work well with noname white PLA used in the smiling owl with Palette 2. But when I selected three PLAs that work well together, 19 hour print went fine unattended. Isn’t it nice: