Err: MAXTEMP, Thermal Runaway, random filament ejection oh my!  

Err: MAXTEMP, Thermal Runaway, random filament ejection oh my!

I don't know exactly why all this is happening in one print session, but it may be pointing to faults in firmware hierarchies and frayed sensor wires.

This photo is what I found after being woken by a screaming MK2s. I didn't know it could scream. On the LCD was Err: MAXTEMP.
Thermal runaway was caused by a breeze in first try of this print as it was an unusually cold and windy night caught the heated bed bare. Enclosure was closed for overnight print even though PETG doesn't need gust shielding.

#1, my PEI is not long for this world. Looks like a Pacific Island air strip after a bomb run. So I've been printing all the way forward-right to eek out a little more life before I do the PEI toxic chemical shuffle. This is an accessory to the crime.
Next perp is free play in the wire bundle coming out of x-carriage-body, I removed some zip ties that caught on Mini-RAMBo cover at X-min travel. This yanked on the hot end thermistor wires causing metal fatigue over time exacerbated by intense heat bed corner printing.
Finally, the wire coming out of the thermistor is a single copper strand joining to a normal wire bunch soldered then covered loosely with shrink tube focusing fatigue on the thinner wire off the thermistor.

When disassembled, the thermistor lead detached with the vaguest of tugs. So problem found. Though, knowing that the printer will basically go crazy if your thermistor is flaky, it points to some weak coding on the firmware side as far as failure modes are concerned.

On failure mode lunacy: The specific situation happened prior to "Err: MAXTEMP" in photo. I had a print going down with some blobbing, I attempted to clear the blob, setting print to pause, all worked as planned but then when the print resumed, the filament ejected, printer paused with no readout of error then attempted to continue printing without a loaded filament. So I paused again, started a load sequence and when printer attempted to start once more, it ejected the filament and "dry" printed again. This time, the printer was completely reset, the wasted filament was cleaned off and print was started fresh, boogers were left to their own devices with enclosure sealed as mentioned prior. Then you know what happened later that night when I was asleep.

Corrective measures:
The thermistor leads were soldered back on with double layer of shrink wrap, a new groove was Dremel'd off to reroute thermistor signal wires separate from hot-end lines as well to completely isolate leads from head movement in future so absolutely no fatigue was possible where the line went single copper.

Here you can see the shrink wrapped portion is below the 90° kink up to wire bundle. This means the wire is bent not the copper lead.

Here is the wire routed up with the U-Stepper and Part Fan wires away from hot-end wires.

Although just resoldering the leads would have sufficed, I'm sure my mods will ensure this issue will never be a problem with my printer again. And the pudding?

It is a desiccant holding spool core for a mod of Sterilite 03948604 Ultra-Seal 4.7 Quart Bowl.