poor temp control

[DPL 0]

My Ultimaker2 has been printing poorly, with lots of stringing and underextrusion when using "Normal" quick-print settings. I've been perplexed by this for while, but finally got around to pressing an external thermocouple against the nozzle to have an independent temperature reading. Wit the US's display showing a nozzle temp of 210 C, I sometimes get a reasonable reading (within a few degrees), but sometimes 240C. I know it really is that hot because the PLA starts to smoke. So I've got an intermittent problem. I can see that the overheating can cause stringing, but not clear that it could cause underextrusion. Maybe the temp goes low some of the time.

Has anyone else seen this behavior, and is there a recommended diagnostic method? Is there anyway to check the control circuitry for temp control? I know I can replace the temp sensor and make sure it's seated correctly.

Thanks for any input

[Labern 775]

The old um2 pt100 weren't always very accurate. I had one that was 20deg out and had similar problems like you.

Replaced it and it was all good after that.

This is why the um2+ has an improved pt100 that is a lot more accurate.

If yours is intermittent then it may due to a loose wire, broken wire or the pt100 could be damaged inside.