All tool heads are cleaned between prints. All printers are in enclosures. Enclosures are positive pressure, filtered air with temperature control. All filament is dried before use according to manufacture specifications. All sensors in question have been checked and despite any dirt or grease, cleaned.
Electrically all failing sensors are in working condition. All connections are correct and no loose wires. I can unplug the sensors and move them to the test bench and read them with Arduinos with zero issues. For the nozzle to cube/leveling washers the electrical connection is proven with a multimeter.
Which is why I believe it may have more to do with Cura and firmware than anything else.
I have one file that I can visually see the stepper for the extruder not turning while printing. I personally checked the g-code and I can see the tool change, I can see the feed rate for the extruder. When I run it I can see the extruder not responding. I can stop the print. Rewind to the beginning and start printing and the extruder responds and does the purge, etc. so the stepper isn’t the issue.
I have another file where the extruder keeps turning (this is with the .8mm HS toolhead) but it throws an extruder error. I turn off both run out and (not in front of the printer so I forget the exact setting name) the jam sensor and the file prints without a problem. Load another file, turn everything back on and it prints no problem. At least the files that do not print do not print consistently so you could debug it, IF you had customer service answering emails.
Many of my prior issues were taken care of by a firmware update Brian sent me, so I am holding out hope this is more code in the system than actual physical problems with the printer.