MK3/S + MMU2S loading wrong filament -- problem and solution  

MK3/S + MMU2S loading wrong filament -- problem and solution

 There is a bug in the filament selection process.    I've had this happen several times today.  I design a small, 1-color item and print out a prototype using MMU2/S extruder #1, gray PLA.   In the slicer I chose filament #1 via right click on Prusa Slicer 2.2.0 and select the extruder #1.   Slice, make gcode, send to printer.  Item prints OK. 

Then for the final version I want black PLA which is loaded in MMU2/S position #3.   Back in Prusa Slicer I right click the item, select extruder #3, re-slice, save as a new file, send to printer then print.  I double check in advanced settings that only extruder #3 is used for this small item.  Same item, but now a different file name using extruder #3 instead of extruder #1.

The MMU2/S filament selector travels to slot #3 BUT LOADS FILAMENT #1.  Filament comes straight out of the MMU2/S into the air and just keeps coming.  It thinks its sending it down the tube to the extruder nozzle but it is going into the air because the filament selector is down at position #3. 

After many attempts at resetting, cycling power, reslicing, making more files, the only way I  cleared the error was to use MK3/S front panel, select     [Load Filament to Nozzle},     [Filament #1] ,      [Unload filament]. 

Then,    [Load Filament to Nozzle] , [Filament #3]

Then the next print went okay and it started printing right away.    I have latest firmware loaded for MMU2S and i3MK3/S as of this date (Dec 2nd, 2020)    fw 3.8.1 loaded.  OctoPrint 1.4.2 on a Raspberry Pi 3b+