Thermal Runaway and Temperature Drops

Thermal runaway is a safety feature designed to prevent printer accidentally catching on fire in the case of dislodged hotend thermistor. If this feature wouldn't be enabled and thermistor dislodged, electronics would get lower temperature reading and by trying to reach the target temperature, heater would reach dangerous temperatures and printer would potentially become fire hazard.

Thermal runaway is configured to shut down the printer when temperature drops more then 15°C for more than 45 seconds. If the temperature reading doesn't recover in the set time period, printer will shut down and display Thermal Runaway error.

All MK2 heaters are tested to be able to run at 200°C with print cooling fan at 100% (set in Slic3r Filament - Cooling)Higher temperature materials must have set the power cooling fan power lower, for example PETG is set for 240°C and fan at 40%. This is generally balanced because higher temperature filaments set more quickly. 

 When you get thermal runaway error try:

  1. Check the ambient temperature. Suggested ambient temperature is 20°C or more. Unheated garage in winter might cause thermal runaway as heater doesn't have enough power when the print cooling fan kicks in.
  2. Visually check the thermistor if it is still securely placed in the heater block body.
  3. Check the thermistor cable connector on the back of the X-carriage if it isn't loose. This is only an issue with E3D V6.1 with cartridge thermistor. See below.
  4. Check the fan power settings aren't too high in your slicer of choice and your material temperature. (100% fan is only doable with nozzle temperatures around 200°C with the V6.1 hotend. See below.)
  5. Check if the fan shroud isn't damaged. In some rare cases users bent it while removing the object or during assembly. This might cause the airflow to be different than originally designed and aiming directly at the heater block. The heater can't be powerful enough to overcome this for safety reasons.


Temperature Drops During the Print

 If you see drop by 5°C - 10°C when fan is switched on (especially when the printer transitions from the 1st to the 2nd layer) that is normal and nozzle temperature should recover rather quickly as the PID regulation kicks in.

E3D V6.1 hotend update

You can easily recognise the E3D V6.1 by its cartridge thermistor.

E3D Sock - During the testing we didn't find the E3D sock to be very good, it tends to fall off and cause print failures and even damage the printer. Failed prints can get stuck to the nozzle and break it. However so if you want to test them, we do have them in stock and we can add it for free to your next order, just leave a note.

Thermal Runaway because of the thermistor connector. New connector can become loose over time, try reseating it. If it doesn't help, the small thermistor wires got broken inside the connector where it is crimped, you best bet is to cut away the connector all together, splice the wires and solder them together. Our manufacturing process is being adjusted (11 Oct 2016) so new printers will come like this.

Temperature graph with loose thermistor connector and after the fixTemperature graph with loose thermistor connector and after the fix

Thermal runaway because of print PLA print settings. We were increasing the print temperature to 215°C make the print stick better to the PEI but failed to set the proper temps of 200-205°C for the later layers. This was not a problem on printers with old V6 heater block. But new heater block version is bigger and therefore has bigger surface area to get cooled by the print fan. Please adjust the Other layers to 200°C as on the screenshot. This will be adjusted in the drivers 1.7.5 drivers.


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