Sneaky failing thermistor masquerading as a clogged nozzle

I’ve been fighting clogging for a while. Some prints work, many don’t. The same print which worked will clog on the next print. And print quality has been kind of inconsistent. Bad filament, I thought? But switching didn’t resolve the problem.

Today, after a frustrating series of attempts to fix clogging, I finally decided to try switching from an all-metal hotend to a PTFE-liner-to-the-nozzle setup to see whether heat creep was my problem. (I have both styles of heat break for this setup, I just needed to cut a new piece of PTFE tube a different length.)

As I started to test with the swapped nozzle, Klipper errored out because the nozzle exceeded 400°C. Funny, it didn’t feel like it was that hot! I worried about PTFE fumes. At least we don’t have pet birds. Then I watched the temperature report keep climbing, up past 600°C. That seemed unlikely. I waved my hand near the hot end and it didn’t feel hot. I gingerly brushed the sock. It was only warm. I removed the sock and discovered I could touch the hot end.

I wonder how long this thermistor has been going bad?

The last time I had to do maintenance on the hot end, the cartridge had fused so thoroughly to the heat block that the heat block was damaged when I finally got the cartridge out. No problem, I was interested in swapping it out for a copper heat block I had already purchased anyway. That’s when I discovered that the copper heat block (as well as another aluminum volcano heat block I had) had no support for 3mm thermistor cartridges, only bare thermistors. I happened to have one bare thermistor in my spares and installed it, grumbling.

I think my hate for the bare thermistors has been validated. I had no idea that my clogging problem was probably just the nozzle being too cold, inconsistently, probably due to damage to the delicate thermistor, even though I tried to be very careful attaching it!

Once again thankful that machining is another one of my hobbies, I drilled a #32 hole (drills tend to drill oversize, and that was closest drill size under 3mm) in the heater block. I then drilled a few mm in with a 4.2mm drill so that the thermistor cartridge wouldn’t interfere with the shoulder on the heat break. It was behind the old thermistor screw hole, and there were a few threads there, so I was able to put a small grub screw in place to hold the thermistor cartridge in place.

After four failed starts while printing the first layer, I now successfully finished a 1.5-hour print.


What printer was it?
My E3’s have the tiny bare thermisters, they look so fragile.
A few months ago mine started dying at the third layer +/- then it would make plastic Brillo…
Thus the start of my whole revamp of an Ender 3

It’s my SK-Tank kit, on which I installed an XCR3D XCR-BP6 heat sink and a 3DSWAY copper Volcano block. The bare thermistor that died was from a Cyclops kit I once planned to use and then abandoned, and the cartridge thermistor was from an XCR3D set with 24V 50W heater cartridges and thermistors.

I use an XCR3D XCR-BP6 heat sink and the BP6 heater block it comes with on my other printer. That takes the 3mm thermistor cartridge natively.

Funny you bring up this problem. I just replaced the wheels on my Y axis and replaced everything in the feed system. I too am having inconsistent prints after never having a problem. I just ordered a new hot end and we’ll see if this solves the problem. When I look at my temperature graph, it is rock steady but I think I may be running too cold also.

In healthcare we always preach never trust your monitors and always put your hands and eyes on the patient!


I did that once with a hotend and now I use a thermal gun to tell me how hot it is first.


I did have a patient try to bite me once! :tooth:


What that was cheeky! :tada:


After reading these posts I orderedCreality 3D Printer NTC Thermistor Temp Sensor 100K for Creality Ender 3/Ender 3 Pro/Ender 3 Neo/Voxelab Aquila (1m/39.4inch)
They arrived Tuesday, and as I was putting one on I found that I had crushed the previous wires by over tightening the screw when I installed my MicroSwiss hotend… Neanderthal with tools… 1 more piece of the chain replaced.


Ugh. The new thermistor is now reading not particularly accurately


So I grabbed another thermistor and installed it. You can see the period while it is disconnected, and then when I re-connected it, it was slightly above room temperature from me handling the cold hotend heater block, and then the temperature stabilizing towards the bed temperature.

My luck has definitely been thin on this printer lately. This is also the printer on which the inductive Z probe failed.

I had to remove the heat break from the heat sink and unscrew it from the heat block to get the thermistor installed, then put it back together, so I get to re-calibrate Z. Again. :sob:

that looks terrible for room temp sensing. Noise on the ground plane or some noise in the metal can of the cartridge(probably should be electrically isolated though)? What a pain.

I think the wire harness became intermittent. You can see from the second graph that with only the substitution of a new thermistor the temperature became stable again.

The heater and thermistor cartridges should be isolated. This is a 24V system, so if it were leaking it would be putting 24V on thermistor sense, which, probably would be bad for the control board… But that clearly wasn’t happening; when the hot end heated up it was still noisy (but not as noisy) and because of the wrong reading at room temperature, it was probably also too cold, just as it had been before.

Ah, so your thermistor change wasn’t just the sensor but was the sensor and long wires attached. Got it. I have built most of my machines so I think of the thermistor as a little sensor and the wires separate. Maybe the ground side was making contact with some other metal surface or something was physically broken like you mentioned, ie wires.

The first time I changed it, the thermistor had long wires directly attached, and I replaced it with a variant with short wires plus a harness. It’s a pain routing wires to the print head.

The second time I changed it, I changed only the cartridge and its short wires, and left the rest of the harness in place.

The problem in each case was at the thermistor.

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