Loose fan bolts can cause stringing, blobs, and eventually, a thermal alarm
New Mk4 for Christmas. Been very happy, but some growing pains. I had to loosen the fan bolts a bit in order to pass the calibration that first day, and for a pre-built machine, I don't know why that was so.
In any case, printed great at first, but last few days, have seen a lot of stringing, and since yesterday, some blobs of melted filament on prints. This really came to a head today with a rather simple print that had some big blobs and lots of stringing.
I eventually received a thermal warning, implying my thermistor was bad. I thought I might have done something cleaning the nozzle of old filament because the wires are close, but I'd read the warnings from Prusa, and did a careful job. However, the Mk4 was detecting changes in temp, so reset the printer and re-ran calibration, and the nozzle failed for heat, asking me if I was using a shroud (which I wasn't).
TLDR: I eventually found that the nozzle fan was loose enough to rattle back and forth. As soon as I used the Hex wrench to tighten that back up properly (I guess I left it too lose from the earlier calibration) the calibration passed. Even better, prints that were poor and full of stringing before are now clean. Lesson for me is do not discount even the tiny effect of a misaligned fan, as the thermal control is so critical that it can cause both calibration and print issues.
Documented for community.