Prusa Error 13204: Thermal Runaway on the Hotend
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Prusa error 13204 is thermal runaway on the print head: the heater is being driven but the temperature reading is no longer tracking it. The firmware shuts everything down. This is a safety mechanism, not a nuisance message, and it should never be disabled — an unmonitored heater cartridge is a genuine fire risk.
Thermal runaway differs from a preheat failure in an important way. A preheat error means the hotend never reached target. Runaway means it was working and then the sensor stopped agreeing with reality — which is the more dangerous of the two, because the heater may still be delivering power.
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The three causes, in order
- Thermistor not seated in the block. If the sensor has worked loose it measures air rather than metal, reads low, and the firmware drives the heater harder. Reseat it and check its grub screw.
- Cable break at the flex point. The print head loom bends on every layer. A partial break produces exactly this pattern: fine at rest, wrong while moving.
- Airflow on the block. A part cooling duct aimed wrong, or a missing silicone sock, can cool the block faster than the heater compensates, which the firmware reads as divergence.
What not to do
Do not comment out the thermal runaway protection, and do not raise the timeout to make the message go away. Both remove the only thing standing between a failed sensor and an unregulated heater. If you find advice to do either, it is bad advice.
After you fix it
Before restarting a long job, confirm the reading is stable: a short test print tells you more than a cold preheat does.
Run a short print and watch the temperature graph rather than the part. A reading that wobbles by more than a couple of degrees at steady state means the sensor is still not seated properly, even if the error has not returned yet.
Related codes
13202 is a preheat failure — never reached target. 13201 is the same on the bed. 13213 is the control board overheating, which is electronics rather than hotend. On other models this error is 12204 (MINI), 31204 (CORE One), 35204 (CORE One L) or 23204 (MK3.5).
If the reading jumped rather than climbed, you are looking at 13206 instead — a shorted sensor, not a runaway heater.
Prusa Error 13204 FAQ
Is thermal runaway dangerous?
The condition it detects is. That is why the firmware stops the printer rather than warning you. Fix the cause; do not work around the check.
Can I disable thermal runaway protection?
Technically it exists in firmware, but you should not. It is the only automatic protection against a heater running with a failed sensor.
Why does it happen mid-print and not at startup?
Because the usual cause is a cable that only breaks contact in certain head positions. At rest during preheat it reads fine; twenty layers in, it does not.
Does a loose silicone sock cause thermal runaway?
It can contribute by letting airflow cool the block unevenly. It is worth replacing, but check the thermistor seating and wiring first.
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