Mysterious Missed Extrusion

[mushroombrew]

I am running a Creality CR-10 V2, modified with an E3D Hemera hotend/extruder and am having some mysterious missed extrusion at what appears to be regular heights along the print above about 30mm up. Never any problems down at the bottom. No signs that the filament is jamming/tangling up. I am using Cura slicer and am printing in Overture "easy nylon." 0.2mm layer height with a 0.4mm nozzle, 250 degree print temp. It only seems to happen when I'm out of the room, and I'm beating my head over the wall over this one. Can't figure out why it's doing it. I'm beginning to wonder if it's a problem with the hemera itself, but I don't know what could be causing it. Anyone seen anything similar?

[LePaul]

Well you ruled out one thing i suspected ... filament tangle. Usually a random under extrusion could be an indication of a partial clog.

Are you printing within an enclosure?
Filament dry?

Id think if it was an issue with the Hemera you'd have issues all over.

Let's start with the cheapest parts first...maybe a new nozzle?

[mushroombrew]

Thank you for your reply. I think I figured it out. My thermistor was in the process of going out. I noticed while preheating the nozzle some time after my original post that it was heating extremely slow. Then, the temperature instantly shot up 25 degrees C and triggered the thermal runaway protection. That seemed more like a hardware issue to me than a software bug. I happened to have a spare thermistor on hand, and replacing it fixed all problems. I think that during the prints, it was returning erratic temperature readings, causing missed extrusion at certain points in the print. So yeah. New one on me. Never had a thermistor go out before that. I think it's my fault though. I was printing at 270C several months back in an attempt to get better layer adhesion on some carbon fiber Nylon that I have. It helped, but the stock thermistor is only rated to 250C. I likely damaged it during that process. I later bought E3D's "high temperature" thermocouple/thermistor and the accompanying amplifier board, but I can't figure out how to get it to work with my CR-10 V2. I am designing/printing orthotic leg braces for medical use. They're called "FIDIM" if anyone is interested or needs a pair. They're on Cults, MyMiniFactory and Thingiverse. They're also free. Nylon seems to be an ideal material to use in making them, but some variants of it can be challenging to print under 250C.

[LePaul]

Ah yes if your temp sensor is all over the place, that could explain that, for sure