When the Original Prusa MK4S throws an error, the screen shows an error name, a short description, and a five-digit code, plus a QR code you can scan with your phone to open the matching article on help.prusa3d.com. This is the same advanced error system Prusa introduced with the MK4, and help.prusa3d.com is the primary source for every code below: it maintains a dedicated “QR Error codes” section under the MK4S product pages, separate from the MK4 pages, even though the underlying system is shared hardware-family-wide.
Each code follows the pattern XXYZZ: the first two digits identify the printer model, and the remaining digits identify the fault. On the MK4S, that model prefix is consistently 26. The MK4 uses 13, the CORE One uses 31, the CORE One L uses 35, the CORE One INDX uses 36, the MK3.9S uses 27, the MK3.9 uses 21, the MK3.5S uses 28, the MK3.5 uses 23, the XL uses 17, and the MINI uses 12. This means a huge share of MK4S faults are literally the same failure, on the same firmware base, as the MK4 version, just with a different two-digit prefix on the screen. Below are the fault groups you will actually see, grouped by subsystem, with the exact MK4S codes as documented by Prusa.
Heating and thermal errors
The largest group of MK4S codes covers the hotend and bed heaters. Hotend preheat error #26202 appears with the on-screen message “Hotend preheat error: Check the print head heater and thermistor wiring for potential damage,” and points to a heater cartridge or thermistor wiring fault on the Nextruder. Bed preheat error #26201 is the equivalent fault on the heatbed. Hotend thermal runaway #26204 and Bed thermal runaway #26203 fire when the printer detects the temperature is not responding the way the firmware expects, which usually means a disconnected thermistor, a failed heater, or a genuinely runaway heating event; treat these as stop-and-inspect faults, not something to retry blindly. Hotend mintemp error #26208 and Bed mintemp error #26207 indicate the sensor is reading implausibly low (often a loose or broken thermistor connector), while Hotend maxtemp error #26206 and Bed maxtemp error #26205 indicate an implausibly high reading. The Nextruder’s heatbreak has its own pair of checks too: Heatbreak MINTEMP Error #26211 and Heatbreak MAXTEMP Error #26212. For all of these, start by reseating the hotend and bed thermistor connectors and checking the wiring harness for pinches or fraying before assuming a part failure.
Loadcell and probing errors
The MK4S, like the MK4, uses a loadcell in the Nextruder for first-layer calibration and stuck-filament detection instead of a separate probe. When that system misbehaves you will see Loadcell Timeout #26528, Loadcell Measure Failed #26526, Loadcell Not Calibrated #26523, Loadcell Bad Configuration #26527, Loadcell Tare Error #26524, or Loadcell Tare Failed #26525. These generally point to nozzle contact during probing, debris on the nozzle or sheet, a loose Nextruder connector, or a printer that has never completed its loadcell calibration step in Settings. Re-running calibration and checking for a clean nozzle and print sheet resolves most of them.
Filament and extrusion errors
Stuck filament detection #26101 is generated by the same loadcell hardware, this time watching for abnormal extrusion force, which can mean a clog, a tangle at the spool, or filament grinding against the extruder gear. The MK4S also has a dedicated Fil. sensor troubleshooting #26114 article and a Maintenance warning #26117 that reminds you to service wear parts (nozzle, extruder gears, sheet) on a schedule rather than waiting for a hard failure.
Connectivity and electronics errors
The MK4S’s WiFi module (ESP) generates three distinct fault codes: ESP Error #26504, ESP Error #26505, and ESP Error #26506, plus ESP not connected #26533 when the module cannot be reached at all. These are almost always resolved by a power cycle or, if persistent, a firmware reflash; they are not indicators of a mechanical problem. Separately, Connect Registration Failed can appear while setting up Prusa Connect on the MK4S (as well as MINI/+, MK4, MK3.9/S, MK3.5/S, and XL), and is a configuration issue rather than a hardware one.
Firmware, file, and system errors
A cluster of codes covers firmware update and file handling: BBF Initialization Failed #26532, BBF Allocation Failed #26531, and Unsupported BBF version #26614 relate to problems processing a firmware update file. File system error #26613 and Unknown file system #26703 point to a corrupted or incompatible USB drive or SD card, best fixed by reformatting to FAT32. Signature verification failed #26606 means the printer rejected a firmware or file because it failed an integrity check, usually from an incomplete download or a non-official build. Marlin Request Timeout #26530 and Heaters Timeout #26817 indicate a communication stall between the printer’s boards. Homing error Y #26305 is specific to the MK4/MK4S/MK3.9/MK3.5 mechanical layout (it does not appear on the CORE One or XL code lists) and points to an obstruction or belt/motor issue on the Y axis.
Blue Screen of Death (Internal Error #26538)
For rare cases that fall outside the red QR error screens, the printer shows a “Blue Screen of Death” (renamed “Black Screen of Death” from firmware 6.5.1 onward, keeping the BSOD acronym), logged as Internal Error #26538 on the MK4S. Prusa’s official guidance is not to try to interpret this code yourself: save a crash dump from Settings, then System, then Save Crash Dump to a USB drive and send it to Prusa support along with your printer’s serial number, firmware version, and a description of what happened right before the crash. This is one of the few cases in the whole system where opening a support ticket, rather than following a troubleshooting article, is the documented next step.
How the MK4S relates to the MK4, CORE One, and MK3S+
Stated plainly: the MK4S does not have its own separate error-code language. It runs the same five-digit, QR-linked system Prusa built for the MK4, sharing the exact same fault catalogue (preheat errors, thermal runaways, loadcell faults, ESP errors, BBF and file-system errors, and so on) with only the printer-prefix changing, 26 for MK4S versus 13 for MK4. Its dedicated “Print Error Messages” page on help.prusa3d.com does not list any codes of its own; it simply points back to the QR Error codes article. What genuinely sets the MK4S apart from the MK4 is not software, it is the Nextruder revision: the MK4S ships with an updated hotend and extruder assembly, part of Prusa’s broader hardware refresh across the MK4 family, rather than a new fault-reporting scheme. If you are diagnosing an MK4 (not MK4S) printer, read the full five-digit code and prefix on your screen and use the Prusa MK4 Error Code Index instead, since the codes there use the 13XXX prefix. If your printer is a CORE One, which shares the same loadcell and Nextruder-based platform under a different frame, check the Prusa CORE One Error Code Index for its 31XXX-prefixed codes. Older MK3S+ printers use a different, non-QR error message system entirely, covered separately in the Prusa MK3S+ Error Message Index.