Hot End Overheating

[dluby]

Hey guys, so I printed a few ABS parts yesterday no problem. I went to do the same thing today and while the extruder was heating up all the sudden it started to smell very strongly of burning ABS and I could see the filament smoking at the tip of the nozzle. I shut it down and then quickly hit the forums to try and figure out the problem. I've tried a few things that were suggested in threads I found searching similar problems, but am trying to narrow down exactly what is causing it.

I originally thought the culprit would be the thermistor in the hot end, but when heating up or even stationary at a temperature it is very consistent (aka doesn't fluctuate wildly) even when I wiggle or play with the wires. It doesn't seem to run away wildly either because I left it at around 200C (or what read out as 200) for awhile and it seemed to stay consistent. However, once I go to even 220 it starts to burn, which is 15 degrees short of where I usually print with no problems.

Can the thermistor's resistance have changed without compromising it completely (hence the consistent readouts still), or is the board or software to blame if maybe the temperature read out is just being ignored and heating continues? Also, the ambient temperature on the thermistor seems accurate to what it has been so it doesn't seem to have an "offset" occurring. I'd love any help diagnosing the exact problem so I don't replace more than I have to. Anyone have any idea what could cause this? Let me know if you need more information and/or things you suggest trying.

[ednisley]

trying to narrow down exactly what is causing it

Save yourself an infinity of screwing around: replace the extruder thermistor.

Thermistors tend to fail by having their fine wires break bit by bit, increasing the total resistance, which causes the controller to see an erroneously low temperature. This tends to be frustratingly intermittent, but that's the root problem.

It's possible a few strands in the thermistor wires inside the braided sheath from the hot end to the controller have broken or that a connector is working its way loose, but (AFAICT) everybody who starts by assuming the problem isn't the thermistor eventually replaces it and the problems Go Away.

For sure, it's not firmware or slicer settings or G-Code or anything else you haven't changed ...

For example(s):
viewtopic.php?f=7&t=3076

[CCRN]

How do you change the thermistor? Does it just pull out of the V4?

[3dPrintingMD]

There are set screws located on the side of the Hot end, in the aluminum block. You might have to do some cleaning to get it free. You can try to gently tap it out from the bottom once you loose the set screw.

[dluby]

Thanks Ed, I appreciate the help! I have a new thermistor on the way and will let you guys know if it works out. Thanks for the tip on removing it as well MD!

[sthone]

I had this happen to me the other day in between prints, first batch ran fine and then during the second set I looked at my octopi on my phone and saw temps of 270. You can see where I went downstairs and wiggled the wire at the end... I'm taking Ed's advice too and just changing it out.

[dluby]

Hey guys thanks again for all the help, got the printer up and running again and everything appears to be working perfectly!

[ednisley]

everything appears to be working perfectly

Looks like I can put another notch in my lipstick case ... [grin]