Thermal Runaway Protection is a safety feature, designed to prevent a 3D printer from accidentally catching on fire as a result of the hotend thermistor becoming dislodged. If this feature were not available and the thermistor encountered an issue, the temperature reading would be incorrect and the electronics would continue to try reaching the target heat levels. In doing so, the heater element would reach dangerously high temperatures and potentially pose a significant fire hazard.

Thermal Runaway is configured to shutdown the 3D printer when the temperature drops more than 15°C for more than 45 seconds. If the temperature reading doesn't correct itself in that time period, the 3D printer will shutdown and display a Thermal Runaway error.

Setting thermal parameters in Marlin firmware

• Marlin Configuration for Thermal Settings

 When you get a thermal runaway error:

1. Check the ambient temperature. The recommended ambient temperature is 20°C or more. An unheated garage during winter months may cause a false thermal runaway error, where the 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.