Prusa error 13604 (No FW on USB) appears on the MK4 and its equivalents are #26604 on the MK4S, #21604 on the MK3.9, and #23604 on the MK3.5. The printer scanned the USB drive for a firmware update file but could not find any valid .bbf file to install.

What the Error Means

The printer looks for firmware update files with the .bbf extension in the root directory of the USB drive. If no .bbf file is present, if the file was placed inside a folder rather than the root, or if the file has been renamed and lost its .bbf extension, the printer reports that no firmware is present on the drive. This is distinct from error 13603 (wrong file size) — here the file is simply not found at all. The error also occurs when the USB drive is formatted with an incompatible file system that the printer cannot read.

Step-by-Step Fix

1. Download the latest firmware .bbf file for your model. Go to prusa3d.com/drivers, select your exact printer model, and download the firmware file. The file will have a .bbf extension.
2. Place the firmware file in the root directory of the USB drive. The file must be at the top level of the drive — not inside any folder. If your operating system hides file extensions, enable “Show file extensions” in File Explorer (Windows) or Finder (macOS) to confirm the file ends in .bbf and has not been renamed to .bbf.zip or similar.
3. Confirm the file name has not been altered. Some download managers or antivirus tools rename downloaded files. Rename the file back to its original name with the .bbf extension if needed.
4. Format the USB drive as FAT32 if needed. The printer requires FAT32 format. On Windows: right-click the drive → Format → FAT32. Then copy the firmware file to the newly formatted drive.
5. Retry the firmware update. Insert the USB drive and navigate to LCD Menu → Settings → System → FW Update, or simply power on the printer with the drive inserted — the printer typically detects the firmware file automatically.

Parts Required

• No replacement parts are required for this fix — this is a file preparation issue

If the firmware file is in the root of a FAT32 drive and the error persists, contact Prusa Support at help.prusa3d.com to confirm the correct file name for your specific printer revision.

Frequently asked questions

What does Prusa 13604 mean?

Prusa error 13604 (No FW on USB) appears on the MK4 and its equivalents are #26604 on the MK4S, #21604 on the MK3.9, and #23604 on the MK3.5. The printer scanned the USB drive for a firmware update file but could not find any valid .bbf file to install.

How do I fix Prusa 13604?

Prusa error 13604 means no valid firmware file was found on the USB drive. Learn how to download and correctly place the .bbf firmware file to complete the update.