Note the exact five-digit code shown on the MK4’s screen, in the format XXYZZ (for example 13204), along with the full error name printed above it and what the printer was doing when the fault appeared: homing, heating, first layer calibration, or mid print. Write down the filament type and nozzle temperature in use too, since that context narrows down the cause before you open the matching article.
MK4 error screens show a QR code plus a text link such as help.prusa3d.com/en/13204. Scan the code or type the link in directly rather than searching Prusa’s knowledge base by symptom, because the MK4 runs its own Nextruder extruder and load cell first layer sensor, and articles for the MK4S and the CoreXY CORE One sit under different code numbers even when the error name reads the same. Bed thermal runaway, for instance, is code 13203 on the MK4, 26203 on the MK4S, and 31203 on the CORE One, so matching the code to the exact model keeps you from troubleshooting hardware your printer doesn’t have.
For thermal runaway codes (13203 for the bed, 13204 for the hotend), check that the thermistor is fully seated before suspecting the heater cartridge: the bed thermistor sits under the gold kapton tape, and the hotend thermistor needs to be pushed fully into the heater block. For homing errors on X or Y (13304, 13305), check for a cable tie, spool holder, or filament loop blocking the axis, and confirm the belt hasn’t skipped a tooth. For a bed preheat error (13201), check that the bed cable connector at the rear of the heatbed is seated all the way in.
Don’t keep restarting a print that stopped on a thermal runaway error without opening the hotend or bed area first. That safety cutoff exists specifically to stop heater damage or a fire risk, and repeating the print without checking the wiring risks reproducing the same fault. Don’t unplug or reseat any connector while the printer is still powered on, and don’t reflash a different firmware version just to make the message disappear before you’ve identified the part that’s actually failing.
If the fix described in the matching article doesn’t clear the error once you’ve checked the wiring and connectors it points to, open a support ticket through help.prusa3d.com rather than posting on the general forum. Include the printer’s serial number, the exact five-digit code, the firmware version shown in the About menu, and a photo or short video of the fault if it’s something visual, like a jam or a loose part.
The QR error code system runs across Prusa’s whole current lineup, but the codes themselves are model specific: the Prusa CORE One error code index covers the same five-digit format on Prusa’s CoreXY machine, where thermal runaway and homing faults carry different numbers than on the MK4. Routine upkeep also heads off a lot of these faults before they show up, and the Prusa CORE One maintenance schedule covers the kind of checks worth running on the MK4 as well. The predecessor Einsy-based Prusa MK3S+ does not use this QR/five-digit system at all, it shows plain text error messages instead.
For the smaller Buddy-board printer in Prusa’s lineup, which shares this same fault framework but drops MMU support entirely, see the Prusa MINI+ Error Code Index. For the MK4S, the Nextruder-revised hardware update that uses the same five-digit system with a 26XXX prefix instead of the MK4’s 13XXX, see the Prusa MK4S Error Code Index.