Prusa error 13603 (Invalid FW Size on USB) appears on the MK4 and its equivalents are #21603 on the MK3.9 and #23603 on the MK3.5. The printer found a firmware file on the USB drive but its file size does not match what the printer expects for its model, preventing the update from proceeding.

What the Error Means

When a firmware update is initiated from USB, the printer reads the .bbf file and checks its size before attempting to flash it. If the file size is outside the expected range for the firmware image, the printer refuses to proceed — this is a safety check to prevent attempting to flash a truncated, corrupt, or wrong-model firmware file that could brick the board. The most common causes are a partially downloaded firmware file (where the browser or server cut the download short), a firmware file intended for a different Prusa model, or the wrong file type entirely being placed on the USB drive.

Step-by-Step Fix

1. Download the correct firmware for your exact printer model. Go to prusa3d.com/drivers, select your model (MK4, MK4S, MK3.9, MK3.5, etc.) carefully, and download the firmware. The MK4 and MK4S use different firmware files — confirm you select the right variant.
2. Verify the file downloaded completely. After downloading, check the file size on your computer against the size listed on the Prusa website or in the download description. If they do not match, delete the partial file and download again.
3. Copy only the firmware .bbf file to the root of the USB drive. Remove other .bbf files from the drive to prevent confusion. Place the firmware file directly in the drive root, not inside any folder.
4. Reformat the USB drive as FAT32 if the issue persists. A file system issue on the USB drive can occasionally cause file size reads to be incorrect. Format the drive as FAT32, then re-copy the firmware file.
5. Try a different USB drive. If reformatting does not help, the drive itself may be reporting incorrect file sizes due to a hardware fault.

Parts Required

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

If a freshly downloaded firmware on a newly formatted USB drive still produces this error, contact Prusa Support at help.prusa3d.com with your printer model and firmware file name.

Frequently asked questions

What does Prusa 13603 mean?

Prusa error 13603 (Invalid FW Size on USB) appears on the MK4 and its equivalents are #21603 on the MK3.9 and #23603 on the MK3.5. The printer found a firmware file on the USB drive but its file size does not match what the printer expects for its model, preventing the update from proceeding.

How do I fix Prusa 13603?

Prusa error 13603 means the firmware file on USB is the wrong size. Learn how to download the correct firmware file and prepare the USB drive for a successful update.