Can thermistors change?

For some reason I'm having to increase the hot end temperaure and print slower than I'm used to, otherwise I get severe under extrusion and the extruder chewing up the filament. I was thinking it was a bad reel of filament but have tried a part used reel that I know was good and get the same issues. So, I can no longer print PLA at 190 or 205 deg C. I have to use at least 220 and 235 deg C is better still and I have to slow the printer down by 80% compared to what I have always used. My gut feeling is that the temperature isn't really what I'm seeing. So, I was just wondering if thermistors can suddenly change their characteristics.

Normal cause for that sort of thing is that the thermistor is escaping from the hot end, a very small gap and the temperature reading lowers a lot.

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Dust
Normal cause for that sort of thing is that the thermistor is escaping from the hot end, a very small gap and the temperature reading lowers a lot.

Cheers - I'll check that.

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Dust
Normal cause for that sort of thing is that the thermistor is escaping from the hot end, a very small gap and the temperature reading lowers a lot.

Wouldn't that cause the actual hotend temperature to be higher than intended? A breakdown in insulation would cause the thermistor to over-read and result in lower than expected hotend temps. When the printer has been unused for several hours the hotend(s) and bed should all give similar readings around room temperature. It's really useful to have a k-type thermometer to double check temps with, and the thermistors are sufficiently delicate that keeping a spare or two is a good idea.

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JamesK

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Dust
Normal cause for that sort of thing is that the thermistor is escaping from the hot end, a very small gap and the temperature reading lowers a lot.

Wouldn't that cause the actual hotend temperature to be higher than intended? A breakdown in insulation would cause the thermistor to over-read and result in lower than expected hotend temps. When the printer has been unused for several hours the hotend(s) and bed should all give similar readings around room temperature. It's really useful to have a k-type thermometer to double check temps with, and the thermistors are sufficiently delicate that keeping a spare or two is a good idea.

Good points. I do have a couple of spare thermistors but it's a real pain to take my Diamond hot end apart so I wanted to check if it's a likely cause before I start. I never though to look at what it reads at normal ambient - I think if it was a long way out that it would have jumped out at me but I'll take a gander when I've finished printing. I had thought that K type thermometers were really expensive so I've never bothered but since you mentioned them, I took a quick look on Amazon and they are quite reasonably priced - think I'll get me one. Thanks for the tip.

Over time the resistance of a thermistor at a certain temperature can change especially if they get overheated or too much current goes through but as others stated it is not making as good of contact with the heater block as before like loose Kapton tape. Hot tubs usually show a lower than actual temperature letting water get too hot. Maybe reference resistor has changed also.

Sorry, should have got back to this before. I failed to mention that I'm using a Diamond hot end. The problem was not the thermistor but a partial blockage caused by heat creep on the unused inputs when printing single colour at slightly elevated temperatures.