Prusa error 13204 (Hotend Thermal Runaway) appears on the MK4 and maps to #26204 on the MK4S, #21204 on the MK3.9, #27204 on the MK3.9S, #31204 on the CORE One, and #35204 on the CORE One L. The firmware detected that the nozzle temperature dropped more than 12°C and the heater could not recover it within 60 seconds during a print.

What the Error Means

The hotend thermal runaway check runs continuously during printing. A 12°C drop that cannot be recovered within one minute indicates the heater has lost significant effectiveness — either because the thermistor is misreading the temperature (causing the firmware to think it is hotter than it is, so it reduces power) or because a physical failure has genuinely cut heating capacity. Common causes include a thermistor not fully seated in the heater block, a damaged LoveBoard cable, a pinched main cable at the xBuddy board, or a blown extruder fuse on MK4/S and MK3.9/S models.

Step-by-Step Fix

1. Power off and cool fully. Allow the hotend to drop below 40°C before touching any components. Working on a hot block risks burns and can damage the thermistor on contact.
2. Check ambient temperature. Confirm the room is above 18°C. Unexpectedly cold air can slow the heater’s recovery rate enough to trigger the runaway threshold.
3. Confirm thermistor seating in the heater block. Remove the silicone sock from the heater block to expose the thermistor port. The thermistor should be inserted completely to the bottom of the port; any gap causes it to read a lower temperature than the block’s actual temperature, creating a feedback loop where the heater runs at reduced power.
4. Inspect LoveBoard heater and thermistor cables. Access the LoveBoard per your model — on MK4/S and MK3.9/S, slide the extruder cover upward; on CORE One, use a T10 Torx driver. Verify both the heater cable and thermistor cable connectors are fully seated with no bent pins or signs of heat damage.
5. Check the LoveBoard main cable at the xBuddy board. Trace the cable from the extruder to the board and look for areas where the cable is pinched by an edge or bracket, which can cause intermittent resistance under movement.
6. Inspect the extruder protection fuse (MK4/S, MK3.9/S). Open the xBuddy box and check the extruder fuse. A blown fuse fully disables the heater. Test for continuity and replace if open.
7. Measure resistance with a multimeter. Test heater resistance (typically 14–16 Ω) and thermistor resistance (roughly 100 kΩ at room temperature). Out-of-spec values indicate the failed component.

Parts Required

• Replacement NTC 100k thermistor (if resistance is incorrect)
• Replacement hotend heater cartridge (if resistance is out of spec)
• Replacement extruder fuse (MK4/S, MK3.9/S — verify correct rating in Prusa documentation)
• Silicone sock for heater block
• Digital multimeter
• T10 Torx screwdriver (CORE One)

Contact Prusa Support if the error returns after completing these steps, especially if your multimeter readings are within spec but the error persists.

Frequently asked questions

What does Prusa 13204 mean?

Prusa error 13204 (Hotend Thermal Runaway) appears on the MK4 and maps to #26204 on the MK4S, #21204 on the MK3.9, #27204 on the MK3.9S, #31204 on the CORE One, and #35204 on the CORE One L. The firmware detected that the nozzle temperature dropped more than 12°C and the heater could not recover it within 60 seconds during a print.

How do I fix Prusa 13204?

Prusa error 13204 means the nozzle dropped more than 12°C without recovery in 60 seconds. Fix thermistor, LoveBoard, and fuse issues with this guide.