Prusa error 13203 (Bed Thermal Runaway) triggers on the MK4 and has equivalents at #26203 on the MK4S, #21203 on the MK3.9, #27203 on the MK3.9S, #23203 on the MK3.5, #28203 on the MK3.5S, and #31203 on the CORE One. The printer observed the heatbed temperature falling more than 2°C continuously for over 60 seconds while a print was in progress.

What the Error Means

Thermal runaway protection monitors the bed temperature throughout a print. If the bed loses heat faster than the heater can compensate — and that deficit persists for 60 seconds — the firmware shuts down and raises this error to prevent a potential fire hazard caused by runaway heating. The most common causes are a thermistor that has shifted off the bed surface and is now reading ambient air instead of bed temperature, an air draft cooling the bed surface faster than the heater can keep up, or a power cable that has slipped loose from its connector inside the xBuddy enclosure.

Step-by-Step Fix

1. Eliminate cold drafts. Ensure the printer operates in a still-air environment warmer than 18°C. Air conditioning vents, open windows, or fans blowing across the bed are frequent causes of persistent temperature drops, especially on large, slow prints.
2. Verify the thermistor is secured under Kapton tape. Power off the printer and let it cool. Lift the bed and confirm the thermistor is lying flat and fully covered by the golden Kapton tape. An exposed or lifting thermistor reads cold air rather than the bed, causing the firmware to demand more heat and then detect a runaway when temperatures appear to drop.
3. Print multiple small objects simultaneously. On prints with very small cross-sections, the bed can experience localized cooling between layer moves. Adding a second object to the print or enabling a brim keeps the toolhead moving over the heated area more continuously, reducing the thermal delta that triggers the error.
4. Inspect the heatbed power cable connector. Open the xBuddy enclosure and check that the heatbed power cable is fully seated in its connector and has not been pulled loose by bed movement. A partially disconnected cable reduces heater power output.
5. Test cable resistance with a multimeter. With the printer powered off and cooled, measure continuity and resistance on both the thermistor and heater circuits. A significantly elevated resistance on the heater cable indicates a damaged cable that needs replacement.

Parts Required

• Kapton tape (to re-secure thermistor if adhesive has failed)
• Replacement heatbed thermistor (if resistance is out of spec)
• Replacement heatbed power cable (if damaged or high resistance)
• Digital multimeter

If the error reappears after these steps are completed, contact Prusa Support with a description of the print conditions and your multimeter readings.

Frequently asked questions

What does Prusa 13203 mean?

Prusa error 13203 (Bed Thermal Runaway) triggers on the MK4 and has equivalents at #26203 on the MK4S, #21203 on the MK3.9, #27203 on the MK3.9S, #23203 on the MK3.5, #28203 on the MK3.5S, and #31203 on the CORE One. The printer observed the heatbed temperature falling more than 2°C continuously for over 60 seconds while a print was in progress.

How do I fix Prusa 13203?

Prusa error 13203 means the heatbed dropped more than 2°C for 60 seconds during printing. Fix thermistor, cable, and draft issues with this guide.