strange thermistor behaviour and shut down in the middle of a print (GEN7)

I was succesfully running my printer for more than a year now, but during the last days a strange thing happens once every 2-3 hours.

during a print the whole printer shutsdown (motors, electronics, PSU)

in the repetier host I get a message, (hotend) thermistor reading error and then I have to reset the board and restart. In the temperature readings of the hotend I get a spike to 520C (which is an error as it goes instantly there), but I dont know if this is the cause of the shutdown or just a symptom.

When I reset my board the temperature reading work normaly and the printer also.

this strange behaviour happens from time to time and without any obvious provocation.


does anyone had similar problems in the past?


I use GEN7 electronics and repetier firmware

Check for a short (most likely) or open wire to the thermistor. Make sure you're moving the X/Y/Z around as you may find it only happens with a bit of movement.

Re: A short. You may have a short between the hot end and one (or both) of the thermistor wires. If the wires are covered with PTFE or kapton, this may have melted/degraded over time and is now allowing things to short out. All it then takes is a bit of movement that "bumps" the exposed wires into something and hey presto, you get a "huge" temp spike recorded.

The firmware has limits built into it for this (which afaik will stop the print). You can turn them off, but I would be hesitant about doing that. If you do, watch the machine closely for the entire print. If it briefly temp spikes and then settles down again, it really points to an intermittent short. MAKE SURE to put the limits back into the firmware and load it back on your board, as it's there to protect your hot end, printer, house, and your life from danger.

Thank you Ceflar, Ill check for a short which is a way better fault than a faulty thermistor,

its a hell to get to the cables again as they are full of capton tape, but your suggestion to look closely for spikes while printing with auto stop disabled is quite a better way to pinpoint the faulty behavior

the short would probably be near the hotend where everything heats up considerably, wont be?

You can measure the thermistor at its connector to the board. The nominal value (100k) is at room temperature. Lower values at higher temperatures.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Waste: Yes it could most likely be near the hot end if it's related to temp.

What sort of thermistor have you got? An axial one (cylindrical, one lead goes in one side, and another lead goes in the other) or radial (both leads come out the same side, usually looks like a bead or tear drop).

With the radial ones, if your insulation moves away from the tear drop/bead shape, you end up with exposed wires that could touch each other, as depending on the thermistor, some of them are very thin.

thanks both of you, I have a radial thermistor but im on vacations currently so I have left the fix for when I come back.

I will post my results then for the sake of further reference by other people