Thermal runaway event when saving Z-offset

[sanchosk]

I have stock SV08.
I had to replace hotend for new one due to failed print ruining my hotend.
After installing the new hotend and calibrating, everything worked fine.
I noticed that during first layer print, the surface is not as "squished" as I am used to - so I adjusted the Z-offset by 0.1mm down.
The result was excellent after 3 prints, so I decided to save the offset.
As I've hit save, the printer should reboot.
Instead, I received notice that there was MCU thermal error - reporting temperature above threshold.
I spent some 3-4 minutes trying to reboot the mainboard remotely (the printer is in my garage), no avail, constant complain about overtemperature.
So I went to garage - to find out my hotend literally smoking, half of the garage filled with nasty smoke and stink and the heater constantly on on the hotend.
I quickly plugged the printer out of the wall.
After 3-4 minutes (ambient temperature below 10 degrees in garage) the hotend was still too hot to boot, another 2-3 minutes resulted in temperature above 380 degrees and falling slowly down.
This seems to be quite critical issue - reboot causes the hotend to get stuck in full heat mode and if the printer was not completely cold (e.g. the Z-offset was saved shortly after pre-heat), the hotend will not have enough time to restart before the temperature runs out of range - at which point it will get stuck and the heater will remain on (!!!).
This is a serious issue, needs urgent attention.
In such cases, when thermal runaway is detected, the printer MUST turn the heater off before refusing to continue operation!

[evelant]

Yes I experienced something similar. I'm not sure exactly what caused it, but the hotend was heated to a normal temperature, I rebooted the printer, and the heater turned on full blast during the boot sequence! It got up to 500+c and scorched the build plate. Luckily I was nearby and noticed something was wrong and shut it down immediately. After it cooled down it hasn't happened again and luckily there was no damage beyond a small scorch mark on the build plate, but it was scary.

This should never be able to happen.

I think the core of the problem is that there is no thermal protection at all while the printer is booting. If some fault turns the heater on it will stay on during the whole boot process which is long enough to cause damage or even fire. The MCU firmware needs some way to monitor temp and heating so it can safety shutdown before the mainboard is booted.

[KiwiHuman]

I had the same thing happen whilst installing the eddy probe upgrade kit, after installing the new firmware my SV08 got stuck with kipper not initialising properly, and the hot end heater was stuck on, I noticed when the hotend was giving off a large amount of smoke.