NOTE: This article assumes that no other outside circumstances are causing the print head to stop mid-print. If this issue occurred overnight, for example, please first verify that there was not a power failure, electrical wall outlet issue/tripped fuse, or that a colleague did not stop the print after you left the office for the day.

How does the thermistor in the printer work?

    We use what is called Negative Temperature Coefficient thermistors in our 3D printers. This means that the higher the temperature of the thermistor gets, the lower the resistance (measured in ohms, "Ω") output is for the thermistor reading. The firmware takes 1 reading roughly ever split second or so to make sure that the thermistor is providing a output. The  thermistor needs to be in circuit to get an accurate reading. When heating, the firmware uses this reading to stabilize a the extruder at a certain set temperature.


What happens when the firmware does not read a thermsitor reading?

    The firmware checks for this thermistor reading as a safety feature. If the safety feature wasn't in place, a faulty thermistor could be reading 0º, and the printer would keep heating a set extruder or bed temperature to an unsafe limit. Therefore, the firmware is set to put the printer in a lock-down mode that we call MIN/MAX Temp Triggered Error. The printer will stay in this lock-down mode only allowing you to home the printer. Program language speaking, only G28 (which homes the printer) is allowed, all G0 or G1 (movement and printing) commands as well as heating commands are ignored. If you try to run a printable .GCODE in either Stand-Alone or Connected to PC mode, the printer's firmware will not allow it until the motherboard is reading a proper thermistor resistance reading again. If you ever see an M999 error, it is telling you that your printer is locked up with a MIN/MAX Temp Trigger Error and will need the g-code command M999 entered in order to reset the error. However, if you manually enter M999 in the g-code terminal while the thermistor is still not providing any reading, the firmware will take that next 0.0625 second reading and send the machine right back into lock-down mode again.

What if it's the thermistor of an extruder that you don't even use?

    Regardless, of whether or not you use the extruder, the printer doesn't want human error to cause a fire - to put it bluntly. If you accidentally set the wrong extruder to heat in the print profiles used to slice a .GCODE or preheat the wrong extrduer, the firmware is designed to lock-down the printer's functionality until the "un-used" thermistor is sending a resistance signal again.

So how do I troubleshoot this on my Bolt?


    The first step is to check and make sure the the hot ends and bed thermistors are connected properly. Also have a look and make sure that the wires are not damaged or loose in the connector. Gently tug on them and see if the wires detach from the connector:


    Also have a look and check the UTP cables running from the printheads to the motherboard and make sure they are plugged in properly (if you take them out and plug them back in they should snap into place).


    Also the printer should tell you on which printhead the error was triggered so that is also something you should look for. 

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