Hi everyone, a week ago my printer (heavily modified Neptune 3) started randomly shutting down in the middle of prints. I come back to a print with the "Klipper reports: SHUTDOWN / Lost communication with MCU 'mcu'" error message.

The printer has been "under construction" for the last couple of weeks, but it has been in varying states of "working" for most of the time - working well enough for me to print the parts I needed to get it back to "fully operational". During this time, the printer never shut down like it is now.

Only once I started making little cosmetic changes did the problem present itself. I was running a known-good print, and I got the above error twice (first time after ~2 hours, second time after ~1 hour) before I got a successful print off of it. This was last week.

After this successful print, I continued other prints with no issues. After a day or two with no problems, an hour long print threw the error at me four consecutive times between 10-45 minutes into the print. This is when I started looking into my klippy log and found some relevant articles citing things like EMF interference, bad power supplies, faulty cables etc. I realized that one of the changes I had made rerouted the printer USB cable right around the Z-stepper, so I rerouted it to how it was originally and immediately managed a successful print. This was 5 days ago.

After moving that cable I had no issues with printing several-hour long prints... until last night. I had been printing all day, then the problem came back. After one print finished, I queued up another print with a plate full of parts, it failed after 1.5 hours. Tried the same print again, failed in 30 minutes. I re-sliced to only a handful of parts to see if I could get those to print before the error occurs, and it's failing 15 minutes into the print.

The printer power supply is the unit that came with the Neptune, and it isn't powering anything besides stock hardware (exception being the SKR mini board), so I don't think it's that. The pi is on a quality unit. The USB cable has been working for a long time so I also don't suspect that, but I'm probably going to buy a new one today just to be sure. I adjusted my enclosure setup so that the Pi and SKR are able to get cool air (at one point had a personal fan pointing at the open electronics box, still failed).

Here is a link to my most recent klippy log (abridged to the start of the last failed print). I'm not very familiar with reading through this and finding oddities, but I do think it's strange that it seemed to load my preheat script in the middle of printing right before the EOF error. (It should be noted that this preheat script was made 1 or 2 failed prints before this most recent one, so it isn't the source of the error as prints were failing before the script was made). If there's anything I'm missing or something else I can try, please let me know!

Edit: While typing this post, I was running the same failed print without filament and both heaters turned off. It ran for about 45 minutes (most recent failure occurred at 12 minutes) so I cancelled the print and started it again with heaters turned on, still without filament. It again ran for about 45 minutes, so I again cancelled it and started the print again, this time with filament loaded. It failed in 5 minutes.

Edit 2: A test print with heaters on and no filament failed after 1h8m. So it isn't an issue with extruding filament.

Edit 3: New cable with the 5v leads taped off per @SzethFriendOfNimi@lemmy.world's advice. Ran the print without filament until completion. Reloaded the same file with filament, print ran without issue until the 1h14m mark, at which point I tapped my Klipperscreen device to wake up the screen, and as soon as it displayed the status, the printer errored out. This can't be a coincidence, can it? Whenever the print goes unmonitored for a long time, it fails as soon as I do something (load mainsail, turn on the klipperscreen) to check the status of it.