Guide · Heating, power and control
Thermal runaway protection: what it does and how to confirm it
What Marlin, Klipper and Prusa say their heater protection does, what a MAXTEMP error can mean, how PID tuning relates, and why you never switch it off.
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Short answer
Thermal runaway protection is firmware that shuts the heaters off when a temperature fails to rise, or drifts too far from its target for too long. Marlin calls it one of its most vital safety features, Klipper enables it automatically for every heater, and Prusa's recent printers stop with an error. If it trips, stop printing and contact your printer's maker. Never disable it.
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What it is
A printer's firmware reads the thermistor and switches the heater to hold a target temperature. If the sensor falls out of its block, a wire breaks or a heater fails, the firmware can keep heating without the temperature behaving as expected. Thermal runaway protection watches for that and shuts down.
Marlin's documentation calls thermal protection one of its most vital safety features, shutting down heaters when a temperature fails to rise or drifts too far from target for too long.[1] The errors it shows are "Heating failed" and "Thermal runaway".
Marlin's defaults
Marlin's default hotend protection uses a 40-second period with 4 °C hysteresis, and halts the machine if the temperature fails to rise 2 °C within 20 seconds after a target increase.[1] Bed defaults differ (20 s / 2 °C, with a 60 s / 2 °C watch). These are Marlin's documented defaults: your printer's maker may ship different values.
Klipper: automatic heater verification
Klipper automatically enables heater and temperature-sensor verification for every configured heater; the verify_heater section only changes its defaults.[2] Klipper also shuts down if the measured temperature leaves the configured min_temp to max_temp range.[2] Its defaults include a check time of 20 seconds for extruders and 60 seconds for the heated bed. Do not loosen these values to silence an error.
Prusa: the 12 °C / 60 s rule
Prusa's CORE One, MK4 and MK3.9 families stop heating and show a hotend thermal runaway error if the nozzle temperature drops by 12 °C and does not recover within 60 seconds.[3] The error code differs per model, so search Prusa's help site with the exact code your printer shows.
| Firmware | What triggers a stop (documented) |
|---|---|
| Marlin (defaults) | Hotend: 40 s period with 4 °C hysteresis; halts if it fails to rise 2 °C within 20 s after a target increase |
| Klipper | Verification on every heater; also leaving the min_temp to max_temp range |
| Prusa CORE One, MK4, MK3.9 | Nozzle drops 12 °C and does not recover within 60 s |
MAXTEMP errors
A MAXTEMP error means the reading went above the allowed maximum. Marlin's documentation says it usually means shorted sensor wires but can also indicate a heater MOSFET or relay stuck on.[1] Both causes are electrical faults. This is a diagnosis to hand to the maker's support, not a repair to attempt yourself.
PID tuning is separate, but linked
Marlin configures thermal protection through separate options from PID tuning, so a PID tune is not a substitute for thermal protection.[1] They are still linked: Marlin says a thermal runaway error may indicate poor PID tuning, and that correct tuning prevents overshoot, which it calls a safety hazard. After a hotend or heater change, see PID tuning after a hotend swap.
How to confirm it is on
- Klipper printers: verification is automatic for configured heaters; ask the maker if its config overrides the defaults.
- Marlin printers: the option is set when the firmware is compiled. Ask the printer's maker to confirm their build enables it for the hotend and bed, and use their firmware updates.
- Prusa printers: the protection is part of Prusa's firmware for the families above.
We give no procedure for provoking a fault to test it; that belongs to the maker.
Next
Questions
Does my printer have thermal runaway protection?
Klipper enables heater verification automatically for every configured heater, and Marlin documents thermal protection as one of its most vital safety features. Prusa's CORE One, MK4 and MK3.9 families show a hotend thermal runaway error. Ask your printer's maker to confirm it is enabled on your unit.[2][1][3]
What does a MAXTEMP error mean?
Marlin's documentation says a MAXTEMP error usually means shorted sensor wires but can also indicate a heater MOSFET or relay stuck on. Stop printing, switch the printer off and contact the maker's support.[1]
Can bad PID tuning trigger thermal runaway?
It can contribute. Marlin configures thermal protection and PID tuning separately, but its documentation says a thermal runaway error may indicate poor PID tuning, and that correct tuning prevents overshoot, which it calls a safety hazard.[1]
Can I disable thermal runaway protection to stop the errors?
No. The error is the protection working. Find the cause with your printer maker's support instead of loosening or disabling the check.