Thermal Runaway Error: What It Means and How to Fix It

A thermal runaway error means the printer stopped heating because the temperature did not behave the way the firmware expected. Most often the cause is a loose or damaged thermistor or heater connection, or a heater that is being cooled faster than it can heat. The fix is to power off, let everything cool, and check the wiring and the sensor first. Firmware makers add this protection because a sensor that reads too cold can make a printer keep heating. Treat the error as a real safety stop, not a nuisance to switch off.

What a thermal runaway error means

Prusa describes thermal runaway as a safety feature that cuts heater power if a cable malfunctions. If the thermistor comes loose, the electronics read a falsely low temperature and may push the heater harder to compensate. Marlin’s configuration file says thermal protection “provides additional protection to your printer from damage and fire.” Klipper turns heater verification on automatically for every configured heater.

Different firmware words the error differently, but the idea is the same: the heater was on, and the measured temperature did not rise or hold as expected.

When the error is triggered

The exact limits depend on the firmware. These are the values the makers document.

Marlin defaults come from the Configuration_adv.h file in the bugfix-2.1.x branch. A given printer’s firmware build may use different values.

Step 1: Check the room and airflow

Cooling can trip the error even when nothing is broken. Prusa’s bed guidance says to confirm the room is above 18 °C and to move the printer away from air conditioning or windows. For the MK3.5 hotend it says the room should be above 10 °C. Prusa also suggests, for small parts in high-temperature filament, printing several at once or placing them away from the center of the bed.

Step 2: Check the thermistor

A thermistor that has moved or loosened is the classic cause. On Prusa heatbeds, check that the thermistor is firmly held under the golden Kapton tape. On a hotend, Prusa says to make sure the whole thermistor cartridge sits inside the heater block with no part sticking out. Klipper’s verify_heater section exists to catch sensor faults, so a thermistor that has come loose is a likely culprit when it fires.

Step 3: Check the connectors and fuses

With the printer unplugged, check that the thermistor and heater connectors are fully seated on the board. Prusa’s steps add that the heatbed power cable should not have slipped out of its screw terminal, and that no fuse should be blown. Prusa notes that some users report tightening the heatbed connection nuts fixed the error. Only open covers if your maker allows it; if not, stop here and contact support.

Step 4: Check airflow onto the heater block

Prusa says to inspect the fan shroud for damage and to confirm it sits correctly, because a damaged or tilted shroud can direct airflow onto the heater block and cool it. A part-cooling fan blowing at the nozzle can cause a temperature dip, which looks the same as a fault to the firmware.

Step 5: Work out whether the heater or the thermistor failed

If the error persists, Prusa describes a hairdryer test for the hotend. Blow warm air at the hotend and watch the temperature on the display. If the reading rises, the thermistor is working and the heater is the likely problem. If the reading does not change, the thermistor is probably the cause. You can also check either part with a multimeter, as Prusa’s guide describes. Bambu Lab’s wiki takes a similar swap-and-compare approach on the H2 series: swap the two heaters and see whether the error follows the heater.

If you are not comfortable with this, contact the maker. Replacement heaters and thermistors are cheap, but mains-powered parts such as a bed heater are a job for the maker’s documented procedure.

Custom firmware: Marlin and Klipper

On Marlin, the configuration file says that if you get false positives for thermal runaway you can increase THERMAL_PROTECTION_HYSTERESIS and/or THERMAL_PROTECTION_PERIOD. Treat that as a last step after the wiring and sensor are confirmed good, and raise the values only a little. Raising them hides a real fault rather than fixing it.

On Klipper, the documentation says heater verification is automatic and can be tuned with a [verify_heater] section. It says it is rare to customize the hysteresis value. See what Klipper is for the background.

Confirm the fix

Plug the printer back in and heat the hotend and bed to your normal temperatures without printing. Wait for several minutes and confirm the readings hold steady. Then start a small print and watch the first layers. If the error returns, stop and contact the maker. For mid-print stops that are not thermal errors, see why a print stops midway.

Related reading

A blocked nozzle is a different problem; see how to fix a clogged nozzle. For general care, the maintenance checklist and 3D printer safety basics cover regular checks of wiring and heaters. Heat-block parts are explained in hotends and nozzle materials.