The possibility of 3D printer catching fire also depends on the version of firmware that is installed ! More recent firmwares are more advanced and have the additional protection measures – like against a thermistor coming off place. For example, below you can find a commit message 43c298a (dated Jun 30 2014) from a Marlin Firmware repository. My cheap Chinese 3D printer (with Atmega 1284P) had a slightly older firmware version installed, so I had to update its’ firmware to enable this “Thermal Runaway Protection”! Always update a firmware of your 3D printer!!!
/*================== Thermal Runaway Protection ==============================
This is a feature to protect your printer from burn up in flames if it has a thermistor coming off place (this happened to a friend of mine recently and motivated me writing this feature).
The issue: If a thermistor come off, it will read a lower temperature than actual. The system will turn the heater on forever, burning up the filament and anything else around.
After the temperature reaches the target for the first time, this feature will start measuring for how long the current temperature stays below the target
minus _HYSTERESIS (set_temperature – THERMAL_RUNAWAY_PROTECTION_HYSTERESIS).
If it stays longer than _PERIOD, it means the thermistor temperature cannot catch up with the target, so something *may be* wrong. Then, to be on the safe side, the system will he halt.
Bear in mind the count down will just start AFTER the first time the thermistor temperature is over the target, so you will have no problem if your extruder heater takes 2 minutes to hit the target on heating.*/