Posted by: @paj418

Prusa support finally got back to me this morning:

"Could you please provide me with the order number related to your original Prusa MK4 printer?The issue might be in the thermistor or heater, which could have send incorrect data to the printers motherboard to process them."

I'll keep this thread updated with any new information I receive.

This answer from Prusa really grabbed my attention.  I've had a thermal runaway problem with my MK4: https://forum.prusa3d.com/forum/english-forum-original-prusa-i3-mk4-hardware-firmware-and-software-help/thermal-runaway-on-some-prints/

I have the 4S upgrade kit, and the MMU3.  I'm in for an adventure it seems.

"We have checked your case with our colleagues from testing department and the silicone socks is required for the MK4S printer with a 360 cooling fan, as it can cool down the heatblock much quicker than the previous side fan shroud.From now on, please keep the sock on the heatblock, with the option in menu enabled and the issue shouldn't appear again.I do apologize for tha initial confusion."

So, I guess they're now requiring a silcone sock for the MK4S new cooling fan. I know a lot of people have had the same thermal runaway problem I've encountered, but some must be able to print without a sock. I'll just continue using the sock since it keeps the block clean and doesn't appear to impact printing.

At this point I tried to “undo” the upgrade to 4s and tried to print, I got worse results. So then I put the upgrades back on to the nexteuder, I noticed that when I was mounting the nozzle fan that there are slots in the fan housing that the mount slides into. I am pretty sure the first time I had the mount on the outside of the fan housing. The screws fit a lot better. I brought the printer back up and Set it back to a mk4s and ran the calibrations. This was when the nozzle temp calibration failed. I ignored it and I was was able finish printing the project that I had started. It actually printed OK, not great, but it didn’t fail.

But I am pretty sure that I fried 2 nozzle temp sensors. When upgrading to the 4s, in my experience, using the sock is absolutely required!  
I ordered a couple more sensors. To be continued…

Posted by: @kyle-randel

Now I am getting nozzle heater calibration errors!!!

Don't forget to change the setting in the printer for sock on/off. I'm not in front of my printer at the moment, but I think it might be under Settings/Hardware/Nextruder, but I could be wrong.

I don't believe the MK4S prints as well as the MK4. On my MK4 I found that when printing with PLA ,it did better without the sock. I tried several times to print without the sock on the MK4S to see if the machine would print better. I got the thermal run away every time.

Once I finish my current project, I plan to remove the high flow nozzle, and go back the regular nozzle. And of course, slice with correct nozzle settings.

Reinstalled the sock and it prints fine again. Since the heater and thermistor seem to work great without the cooling fan running, about the only thing left to try is the heater block. I wrote back to Prusa support and I'll have to wait to see what they say.

Posted by: @bowtie6

Regarding the 'socks'...  I see they are available but darn, placing an order just for genuine Prusa socks is going to be quite expensive to justify the shipping cost to the USA.  Anyone recommend a good quality sock?  Or it doesn't matter?  

I was going to suggest checking PrintedSolid for nextruder socks, but I was unable to find them there. I did find a US vendor selling Prusa branded socks for the nextruder for about $10USD for a pack of 3. I haven't personally used them as a vendor, so I can't attest for the quality of their products. They claim to ship out of Ohio.

"Could you please provide me with the order number related to your original Prusa MK4 printer?The issue might be in the thermistor or heater, which could have send incorrect data to the printers motherboard to process them."

I'll keep this thread updated with any new information I receive.