Temperature error: Thermal runaway, MINTEMP, MAXTEMP or Heating failed

Symptom

The printer turns off the heaters, stops and shows a temperature error: in Marlin and derived firmware, “Thermal runaway”, “Heating failed”, “MINTEMP” or “MAXTEMP” (sometimes with “BED” when it refers to the bed). In Klipper, the board goes into a shutdown state or a heater or sensor error appears.

Likely causes, from most to least likely

1. The thermistor is loose, not fully inserted in the block, or its cable is damaged (often where it enters the block or from the cable bundle moving): the reading fails or jumps.
2. Something cools the heater block or the bed faster than the heater can compensate: the part cooling fan blowing on the block (bent or damaged duct), drafts, or a cold room.
3. Disconnected sensor or broken wire (open circuit): the reading drops below the minimum and MINTEMP triggers.
4. Shorted sensor wires, or a heater MOSFET or relay stuck on: the reading exceeds the maximum and MAXTEMP triggers.
5. The heater doesn't raise the temperature in time (disconnected, loose connector, high-resistance heater or a fault): Heating failed triggers.
6. Poorly tuned PID, for example after changing the hotend, heater, sensor or board.

Checks to tell which one it is

• Identify which protection tripped: MINTEMP (reading below the minimum), MAXTEMP (above the maximum), Heating failed (it doesn't rise as expected while heating) or Thermal runaway (once at temperature, it drifts too far from the target for too long).
• As a guide, with Marlin's default configuration Heating failed triggers if, after raising the target, the hotend doesn't gain 2 °C in 20 s (the bed, 2 °C in 60 s). Each manufacturer can change these values.
• With the printer hot, carefully move the print head and bed along their travel and move the cable bundle without touching hot areas: if the displayed temperature jumps or drops, there's a broken wire inside.
• Check that the thermistor sits fully inside the block without sticking out, and whether the part cooling duct is bent or points at the block.
• In Klipper, the board shuts down if the reading leaves the configured min_temp–max_temp range (similar to MINTEMP/MAXTEMP), and the software also checks that heaters and sensors work properly. Read the exact message in the console.
• If the temperature rises when you haven't asked it to heat, or keeps rising, cut the power at once: a heater may be stuck on.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Turn off, let it cool and check the connectors

   Turn off and unplug the printer and wait until the hotend and bed are cold. Check that the thermistor and heater connectors are fully seated on the board and at the print head, and that no cable is pinched or crushed.

   Check: When switched on, room temperature reads steady and plausible.

   If that does not fix it: Check the thermistor and its cable.

2. Reseat or replace the thermistor

   Check that the thermistor is fully inside the block and secure. If the thermistor or its cable is damaged, replace it entirely: splicing or soldering it changes the reading and is not recommended.

   Check: Moving the print head and cables no longer makes the temperature jump.

   If that does not fix it: Shield the block and bed from airflow.

3. Stop airflow cooling the block or bed

   Straighten or replace a damaged fan duct blowing on the block, move the printer away from windows, air conditioning and drafts, and don't print in a room that's too cold (Prusa, for example, recommends at least 16 °C ambient for its i3 printers). A small drop when the part cooling fan starts is normal if it recovers quickly.

   Check: The temperature stays stable during the print.

   If that does not fix it: Recalibrate the PID.

4. Recalibrate the PID

   If the error appeared after changing the hotend, heater, sensor, board or supply voltage, recalibrate the PID at the highest temperature you plan to use. In Marlin, with M303 (for the bed, M303 E-1) and save the settings with M500; in Klipper, with PID_CALIBRATE followed by SAVE_CONFIG.

   Check: The temperature reaches the target without much overshoot and stays stable.

   If that does not fix it: Check the heater and thermistor with a multimeter or ask support for help.

5. Don't disable or loosen the protections

   Thermal protections stop a sensor fault from turning into overheating or a fire. Don't disable them or change their limits to get rid of the error: find the cause. If the error comes back, contact your manufacturer's support.

   Check: With the cause fixed, the error doesn't come back.

   If that does not fix it: Stop using the printer until the manufacturer's support has checked it.

Prevention

• Check the thermistor, its cable and the part cooling duct now and then, especially after taking the hotend apart.
• Recalibrate the PID every time you change parts of the heating system.
• Keep the printer away from drafts and in a room at a mild temperature.
• Never disable the firmware's thermal protections.

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Configuring Marlin (Thermal protection, temperature ranges and PID) · Marlin
• Frequently Asked Questions (Will the heaters be left on if the Raspberry Pi crashes?) · Klipper
• Configuration checks (Verify temperature, heaters and PID) · Klipper
• Thermal Runaway (i3 series) · Prusa Research