this post was submitted on 17 Jul 2023
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That’s usually a bad thermistor or thermistor autre breaking.
Hmm I hadn't considered that the reading could be wrong, I'll pull apart my hotend later today to have a look at it.
It's a pretty new printer though, I'd be surprised if I have managed to wear down the wire yet, but still I'll have a look.
Check the entire thermistor cable as well as the plug connections for tightness. Preheat the hotend and then wiggle/bend the cable 1 inch at a time all the way back to the main board plug checking to ensure the recorded temp remains constant. (You can also do this with an accurate multimeter checking resistance across the unplugged thermistor if you don't want the printer on while poking around.)
Those aggressive jumps sure look like a thermistor problem, but only when printing in motion, which screams bad cable/connection. Manufacturing errors happen.
Does your Ender 3 still have the screwed-on-cable type of thermistor? These ones break when overtightened, which can happen at the factory.
If the cable and thermistor is turns out to be fine, run a PID tuning.
I had a similar problem caused by loose thermistor wiring, similar to what other people here are suggesting. I used an extra zip tie to secure the wire so it doesn't wiggle during printing, and the issue was fixed for me.
Luck of the draw, could be bad out of the gate, my prusa mk3 had the exact same thing happen less than a month after running it. If you can, add a pinned connection, makes swapping thermistors super easy as a lot of places sell cartridges with microfit connectors.
When I had it happen it wasn't always a problem, was only on certain movements and then it'd error on thermal runaway. I'd say change it anyhow as it's simple to do and you eliminate a cause, best to start with the easy things.