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What happened?

The printer is showing the message "Hotend thermal runaway: Check the print head thermistor wiring for possible damage."

Error name: Hotend thermal runaway

Error code: #28204 (MK3.5S) #23204 (MK3.5)

The error will show if the temperature on the nozzle drops by 12 ºC for an extended period of time. This temperature drop would usually not be an issue, but if the heat is not recovered after 60 seconds the error will happen. In this case, the printer will stop the heating and display the message on the screen.

How to fix it?

  1. Make sure that the ambient temperature where the printer is stable, and above 10 ºC.
  2. Make sure that your hotend assembly is correct, especially if there have been recent interventions or maintenance. 
  3. Visually check the thermistor if it is still securely and correctly installed in the heater block. The whole cartridge should be inside the block, with no part of it sticking out.
  4. Check the fan shroud for any kind of damage, and verify its position. A damaged or tilted shroud can change the airflow and start cooling the heater block directly. 

  5. Make sure the thermistor and heater are properly plugged into the electronics board. Refer to Step 18 from the linked guide to verify the connections. 
  6. Check if the fuses are not blown. 

If the issue persists, the error likely stemmed from an issue with the hotend heater or hotend thermistor. To find out which one of these two is malfunctioning, perform the test described below.

You can also verify if the thermistor or heater is damaged using a multimeter.

Use a hairdryer to blow hot air towards the hotend. In the meantime, check if there is any temperature change on the LCD screen.

  • If there is a positive temperature change it means that the hotend thermistor is working and detecting the temperature change. Most likely, the issue stems from the hotend heater. 
  • If there is no change at all, most likely the hotend thermistor is causing the issue, or at least contributing to it.  
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3 comments

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Bladevane
•
From my experience can I suggest that Prusa revisit their advice and put more stress on the cooling of the hotend block rather than a faulty thermistor or wiring. The printable Fan shroud is a different design to the original and I believe it puts more cooling to the print rather than cooling the hotend. Thank you.
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Bladevane
•
From my recent experience and in my very humble opinion I would suggest that the fan is overcooling the hotend block. I was forced into changing my thermistor, which I set up correctly, and also had to change some of the printed parts - which were slightly different from the orginal parts despite Prusa saying to the contrary. The only part I had not changed was the Fan Shroud. I too received the very unwelcome error 28204 which was resolved by printing a new Fan Shroud with 100% infill so I could fettle it to fit. Once installed my hotend temperature remained within 1 degree C of the set point even with the print cooling fan on 100%. Success. This is a very protracted way of answering your question and my answer is - probably not and yes. Although by now you have probably reached the same conclusion.
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Jim Schrempp
•
I get this error in the middle of a print. I see the temperature displayed going up as the hot end is heated. And the displayed temperature goes down as the hot end cools. Do I still need to do the hairdryer test or does this indicate that the thermistor is working correctly?

Answered: Close observation shows that this happened when the fan was disabled for the first 3 layers and then run at 100% after that. I can see that when the fan came on at 100% the temp dropped. It was printing a flat surface at the time. So much for printing a .gcode from another person.

Does this mean my hot end heater is bunk? Should it be able to stand up to that usage?
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