Hotend preheat error
Got my MMU3 a few weeks ago and added it to my MK 3.9. I was able to print many things without issue and wanted to try adjusting some volume purging when going from black->yellow. So I edited the purging volumes in slicer 2.7.4 to purge a little more when going between these colors. 
But it doesn't work. My Printer goes over to the wipe tower and looks to change filament, but in the process it reboots itself. It comes back with a big red error screen and a lot of beeping saying "Hotend preheat error #21202"
Going to this page it refers me to: https://help.prusa3d.com/article/hotend-preheat-error-13202-mk4-21202-mk3-9_401060
This error is shown when the printer is unable to heat up the nozzle more than 2 ºC for 20 seconds. The printer starts a safety feature, which instantly kills the heating process and displays the message on the screen. The printer then waits for the user to inspect all parts and if needed fix the issue.
There is nothing wrong with my printer. My printer heats up just fine and really fast. This problem goes away when I change back to default purge volumes and instead use global ones (editing the Filament Purge Multiplier in the Printer Settings).
Anyone else experiencing this? is this a Slicer issue?
RE: Hotend preheat error
🙁
Today I was 6 hours into a 9 hour print and it failed again. This time I didn't touch anything in slicer other than to increase the purge volume in the print settings for all filaments.
I put a multimeter on the heater: 16.7 Ω, and on the thermistor at 25 degrees: 100 kΩ
RE: Hotend preheat error
Same problem on my MK4 printer and MMU3. New model with deafult settings can't print -> stuck after 1st filament change -> change works fine, but after that it only travels short distance and restart with Prusa logo -> followed by beeping with mentioned error.. Example of sheep works without any issue.
RE: Hotend preheat error
Like to follow this. Feels same as me
singel print works
mmu peints wont Work after last firmware update. Get preheat error.
RE: Hotend preheat error
Update: talked with support. They said that maybe my heat was too high on the PLA? I was using stock 230 first and 220 other layers... and to try going down to 215?
I have no idea if that helps, it seemed like a wild goose chase to me.
I bought a new thermistor and heater and will try a multi-material print again to see if it happens.
RE: Hotend preheat error
thanks for you update.
i fail all long my print when the Change happen. Tried go back a firmware. No help. Started after last firmware. Single print is no problem.
I am also going to order a whole new block with all I need after I come back home. So very interested in how this get solved.
I also have get som nextruder warnings in my last prints, so many that’s a hint. That’s kind of also one of the solutions in the user guide.
RE: Hotend preheat error
Just finished printing a 3-filament object with different purge volumes with each. No issues and print completed successfully
Since my original post, the following has changed
- Firmware upgrade to 6.0.2
- PrusaSlicer now 2.8.0
- New thermistor and heater
Will update further after many successful prints
RE: Hotend preheat error
Thanks for posting updates.
RE: Hotend preheat error
Hi, thanks for update. I also did update for my mk4 6.0.3 and also slicer 2.8.0 today. Already started printing and everythink looks good now ~1hour (10% of my model) printing without any issue (Stoped on 5% after 1st filament change in the past). I will write review tomorow, but I hope, there was some sw/fw issue. I didn't change anythink on my printer except fw.
I wish you happy printing!
RE: Hotend preheat error
Yes thank you for the update @longer. I too have had no issues recently. I think the old config/prusa slicer was adding in some ridiculous expectations for temperature changes when you modify the purge volumes. It cools down a bit to allow for ramming and getting a good tip, and then it wants the temperature to immediately go back up for the next filament.
So far looks resolved.