Prusa error 13504 (ESP Error) appears on the MK4 and maps to #26504 on the MK4S, #21504 on the MK3.9, #27504 on the MK3.9S, #23504 on the MK3.5, #28504 on the MK3.5S, #31504 on the CORE One, and #35504 on the CORE One L. The xBuddy board attempted to communicate with the ESP Wi-Fi module and could not find or read valid firmware on it.

What the Error Means

The ESP module handles Wi-Fi connectivity on Prusa MK4-family printers. The xBuddy board expects to find bootable firmware on the ESP chip at startup. If the firmware is absent, corrupt, or the xBuddy cannot establish communication over the serial interface, it raises this error. The most common trigger is a firmware update that completed on the main firmware but left the ESP module outdated or partially flashed. Hardware damage to the Wi-Fi board is less common but is checked if reflashing does not resolve the issue.

Step-by-Step Fix

1. Download the latest firmware. Go to help.prusa3d.com/downloads and download the current firmware package for your exact printer model. The package includes both the main firmware binary and the ESP bootloader and firmware files.
2. Transfer firmware files to a USB drive. Copy all required files to the root of a FAT32-formatted USB drive. Safely eject the drive from your computer before removing it — file system errors from unsafe removal can corrupt the files.
3. Insert the USB drive and reboot the printer. Plug the USB drive into the printer while it is on, or power it on with the drive already inserted. Press the reset button located next to the control knob to initiate the boot sequence.
4. Confirm the flashing process starts. The printer should detect the firmware files and begin flashing automatically. If the flashing screen does not appear, press the reset button again; during the boot animation, press the knob once to manually trigger the firmware update menu.
5. Wait for completion. The ESP flashing process can take several minutes. Do not power off the printer or remove the USB drive until the process completes and the printer returns to the main menu.
6. Inspect the Wi-Fi board and xBuddy if flashing fails. If repeated flashing attempts fail, open the printer’s electronics enclosure and inspect the xBuddy board and Wi-Fi module for physical damage, corrosion, or connectors that have come unseated.

Parts Required

• USB drive formatted as FAT32 (for firmware transfer)
• Replacement ESP Wi-Fi module (if hardware damage is confirmed)

If reflashing completes successfully but the error returns after a subsequent restart, contact Prusa Support — the ESP module itself may have failed.

Frequently asked questions

What does Prusa 13504 mean?

Prusa error 13504 (ESP Error) appears on the MK4 and maps to #26504 on the MK4S, #21504 on the MK3.9, #27504 on the MK3.9S, #23504 on the MK3.5, #28504 on the MK3.5S, #31504 on the CORE One, and #35504 on the CORE One L. The xBuddy board attempted to communicate with the ESP Wi-Fi module and could not find or read valid firmware on it.

How do I fix Prusa 13504?

Prusa error 13504 means the xBuddy board cannot read firmware on the Wi-Fi ESP module. Fix it by reflashing the ESP firmware via USB on MK4 and similar models.