Thermal runaway MK3S  

Thermal runaway MK3S

Hi Prusa Support,

I'm having ongoing issues with thermal runaway on my Original Prusa MK3S. During prints, I'm getting a thermal runaway error with beeping.

Here’s what I’ve done so far:

-Replaced hotend heater and thermistor with genuine Prusa parts (installed carefully)

-PID tuning and thermal model calibration 

-Firmware updated to 3.14.1

-Verified connectors, wiring

-ambient temp: 23 c

-Heater works, thermistor shows temperature, but self-test fails on hotend check with “Check the heater/thermistor” error.

This is strange: when I preheat the hotend to 215°C, it reaches and stabilizes at that temperature. But when I turn the part cooling fan on, the temperature drops to 200°C. Then, when I turn the fan off, the temperature spikes up to 240°C.  i guess thats the reason why thermal error happen(on second layer when the fan turn on)

Here is the image after turning off the fan:

Here is the image after turning on the fan:

Self-test error:

Thermal Runaway:

Can you advise on further troubleshooting or next steps?

Thanks,