12401

Moderate

Prusa · FDM 3D Printers

USB Communication Error - Serial Connection Lost

Quick Answer

Error code 12401 on your Prusa FDM 3D Printers means: USB Communication Error - Serial Connection Lost. This is usually fixable — see the step-by-step guide below.

Affected Models

Applies to all models

MINI+MK3.5 MK3S+MK4 MK4S XL

Likely Causes

USB cable is faulty or too long resulting in signal degradation
Host computer USB port entered power saving mode mid-print
EMI interference from stepper motors affecting the USB signal
Buddy board USB connector has a cold solder joint

How to Fix It

Replace the USB cable with a high-quality shielded cable under 1 meter

Disable USB power saving and selective suspend on the host computer

If using OctoPrint, add a USB hub between the Pi and printer to isolate power

Cover the 5V pin on the USB connector with tape to prevent power backfeed (MK3S+ specific fix)

Pro Tip

This fix requires some technical knowledge. If you're unsure about any step, consider contacting support or a qualified technician.

Parts You May Need

Shielded USB-B cable (MK3S+) or USB-C cable (MK4/MK4S)

Powered USB hub

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