Temp fluctuates, then Thermal runway

I hope you can help me with this. It is driving me :peanuts:

What is the problem?

After 15-20 mins of printing, the temperature of the hotend starts to fluctuate, and about 1 minute after that, I get the Thermal runway error. I don't think it actually has anything to do with the time. The issue always appears when the machine is printing the first non-support horizontal layer, indicating it is a underpower issue.

I print PETG at 240C on an ender3 w a .6 nozzle. So a fairly heavy load on the heating element, but I've printed at least 5 kg. worth of filament like that and it has worked just fine until now.

What did you already try to solve it?

After reading about what others tried, I tried:

  1. Added thermal paste around the heating element to better sink heat into the hotend.
  2. Changed the heating element.

None of these produced noticable changes.

Have you tried running in safe mode?


Did running in safe mode solve the problem?

octoprint-systeminfo-20240602210008.zip (415.1 KB)

Systeminfo Bundle

Additional information about your setup

Ender 3, PrusaSlicer, OctoPrint, .6 nozzle, PETG @ 240C

Did you replace the thermistor with the heating element?

Resistors are notorious for breaking down under heat stress.

I would also bet that thermistors are one place a 3D printer company cuts costs.

My Ender 3's bed thermistor started flaking out about 9 months (3,000 hours of print time) after i bought it.

Definitely (I hope), thermistor troubles, change it, then recalibrate the PID settings. PID is a clever bit of mathematics which controls the rate of heating when approaching or at the temperature set point - that’s why there’s an oscillation of the temperature value on the chart. Genius.

Ender 3 power supplies are known to start to "fade out" before they go. I would suggest to get a better power supply (more current). Also check the wiring from the PS to the main board.

could you show us the fluctuations on the temperature graph?

I had a thermistor problem where it was just loose. Might check to see if it is secure.

Yes, here you go:


As mentioned, it always happens when it starts printing vertical parts such as the lid for a box (so when it needs to extrude a lot of plastic)

Surprised no one has mentioned to check the fan speed/direction cooling the nozzle instead of the printed part. If it only happens printing certain features, it looks like when the fan kicks in.

People often use silicone socks to insulate the hotend and that works well.

Yup, I was also thinking oft that. Thats why I wanted to See the graph.

There are asocks that also cover parts oft the nozzle.

I already have a sock on, similar to what is there on the original.

The printer has printed with this setup for years, also with PETG.

Have you checked for proper sit of the thermistor and/or heater cartridge in the heater block?
Also the thermistor may be on the way of giving up.

That could be a tuning issue.

I was thinking about the process of autotuning the other day. Tuning a hot end (extruder) while not under normal load will not produce optimum results. Not to mentioned not all autotuning algorithms are the same. Some just work in theory, and don't produce good results in real life.

I have been considering how to best tune an extruder under a load without dumping extreme quantities of filament in the air.

P.S. There are at least two different types of response modes to deal with when tuning, self regulating and integrating. Tuning methods for each are drastically different. Heating and cooling processes are integrating processes.