Temperature readings suddenly off

[osterac]

Hi all,
I have a duet2 wifi with rrf 3.3 that has suddenly developed a puzzling problem. Mid-print, the extruder started binding up and filament flow decreased. I eventually figured out that this was because the filament was too cold by about 40 degrees, even though it was reading the correct temperature. I replaced the thermistor but the problem remained. I did several PID tunings but it didn't help, though during the second PID tuning the hotend wasn't able to reach the 280 degrees I have it set for. I set the hotend power from 80% to 100% and was able to reach temperature, but 80% has always worked in the past and I used it because I got a warning that the hotend could reach unsafe temperatures in the event that safety features failed.

I guess I could print if I upped the temperature by 40 degrees but I'm not sure if that's a good idea. Any idea what's causing this?

Thanks

Edit:
Another thing I forgot to mention is that the ambient temperature measurement of the hotend thermistor seems to be accurate (when the hotend has fully cooled). It's only when the hotend is heated that I have problems.

[T3P3Tony]

@osterac can you see if the same issue occurs with one of the other temperature inputs? Its weird that its off suddenly by 40C. Did this happen in conjunction with any other change (like firmware upgrade etc?)

[osterac]

@t3p3tony
I have upgraded my firmware recently, but it was working correctly for several prints. I went from rrf 3.2 to rrf 3.3. I tried it in the other temperature input and same issue. I got out the temperature probe for my volt meter and took some measurements right next to the thermistor where it's enclosed in the hotend. I used a little heatsink grease to ensure consistent results.
left side is desired temp, right is measured temp.

40-39
50-48
60-57
70-66
80-75
90-84
100-92
110-101
120-110
130-119
140-128
150-136
160-145
170-156
180-165
190-173
200-181
210-189
220-197
230-206
240-213
250-221
260-228
270-235
280-242
290-249
300-256

[osterac]

@T3P3Tony Since you mentioned firmware I decided to try downgrading to the firmware I was using previously, 3.2. After loading it, DWC reported that I still had version 3.3. I tried version 3.1.1 and version 3.4 beta 5 and it still says I have version 3.3. The DWC version changes though. Could this be related?
I did have the issue where it wouldn't load new firmware because Duet2_SDiap32_WiFiEth.bin was missing but after loading that file the firmware installs, or at least appears to.

Edit: just got it to update. Gonna check it out.

Edit 2: Problem remains with rrf 3.4 beta 5

Edit 3: Still unable to downgrade below 3.3

Edit 4: was able to downgrade to rrf 3.2.2 and then 3.2

Edit 5: Problem remains with 3.2

[T3P3Tony]

@osterac you mention you replaced the thermistor. Did you update the B (&C potentially) values for the new thermistor?

[osterac]

@t3p3tony
Totally forgot about that setting. I found a thread about a thermistor similar to mine and punched in their values and now I get 1% accuracy at 200 degrees and 2.7% accuracy at 300 degrees. Works for me!
Probably was just a bad thermistor.
Thanks so much!