The MK3S+ does not show numeric fault codes on its LCD. It shows short, mostly plain English lines, THERMAL RUNAWAY, MINTEMP, Bed leveling failed, Err: EXTR. FAN ERROR, and so on, sometimes followed by a one line explanation or a prompt asking you to confirm something before the print continues. Prusa’s knowledge base groups these under “Print Error Messages” for the MK3S product line, and that is where everything below comes from. Comments underneath each article are user reports, not manufacturer documentation, so they are excluded here except where they point to something Prusa itself later confirmed.
A word on which printer this is for. The MK3S+ is the last of the classic Einsy based i3 machines before Prusa moved to the MK4/MK4S and CORE One, which use a different mainboard (xBuddy), a load cell based probe instead of a mechanical PINDA sensor, and their own separate error message pages on this site. If your printer homes Z with a physical PINDA or SuperPINDA sensor screwed to the extruder body, and you calibrate it with the “Calibrate Z” and “Calibrate XYZ” menu items rather than a Loadcell test, you are on an MK3S+ (or its close relatives, the MK3, MK3S, and MK2.5/MK2.5S share most of the same firmware and error text). Most of what follows applies across that whole family. Where Prusa’s documentation calls out something specific to the “+” revision, mainly the SuperPINDA sensor that MK3S+ ships with as standard equipment, it is noted.
How to read the message before you do anything else
Write down the exact wording on the LCD before you touch anything. A few of these messages look similar but mean different things, MINTEMP versus MINTEMP BED, Preheat error versus Preheat error (Heatbed), Thermal runaway versus Bed thermal runaway. Getting the hotend/heatbed distinction wrong sends you troubleshooting the wrong half of the printer. If the screen also shows a follow up line like “fixed” or “waiting for reset,” that matters too. It usually means the fault condition cleared on its own but the underlying cause probably has not.
Heating and thermal runaway
Thermal runaway and Bed thermal runaway are triggered by the same safety mechanism: if the firmware asks for more heat and the measured temperature does not respond, it assumes the thermistor has come loose or failed and cuts power rather than risk a runaway heater. Prusa’s firmware trips this when the reading drops more than 15°C for over 45 seconds. The hotend version and the bed version are separate errors, check which one is on the screen. First checks: confirm the ambient temperature in the room is above roughly 16°C (an unheated garage in winter is a common trigger), then visually check that the thermistor cartridge is fully seated in the heater block (hotend) or still taped down with the golden Kapton tape (bed). If it looks fine, move the extruder or bed by hand through its full travel while watching the LCD temperature reading. A cable with a cracked strand will show the temperature jump or spike as you move it. If it does, the thermistor cable needs replacing. Prusa does not recommend soldering or splicing it since the firmware reads its resistance directly. On MK2.5/S printers specifically, Prusa notes the print fan may need capping at 70 to 80 percent on PLA temperatures to avoid nuisance trips. This does not apply to the MK3/S/+ generation.
MINTEMP and MINTEMP BED look similar to thermal runaway but are the opposite direction: the reading has dropped below a hard floor Prusa treats as “thermistor definitely disconnected.” On the MK3/MK3S/MK3S+ (EinsyRambo board) that floor is 10°C for both hotend and heatbed. A fully disconnected thermistor reads a flat 0°C. If the room is genuinely cold, that alone can cause it, Prusa’s suggested fix in that case is a hairdryer on the bed or hotend until it clears 18°C, though materials like ASA or ABS may behave poorly printed cold regardless. If the room is not cold, treat it the same as a thermal runaway investigation: check the connector latch on the RAMBo board, wiggle the cable at the hotend or under the bed, and test resistance with a multimeter if you have one (expect 80 to 125 kOhm at room temperature on a healthy thermistor). The LCD sometimes shows “MINTEMP fixed” after a restart. Prusa is explicit that this does not mean the problem is solved, only that the connection re-made itself for the moment.
MAXTEMP fires when the hotend reads above 305°C, or the bed above 125°C (MAXTEMP BED). Besides a damaged thermistor cable, Prusa flags this happening right after assembly (check your wiring against the EinsyRAMBo guide before anything else) and an occasional glitch where a cold air gust tricks the heater into overshooting briefly, in which case just restart and watch it. Some materials printed hot, PC or PC-CF at 285 to 305°C in particular, have triggered this on stock firmware. Prusa’s own support has pointed owners toward newer firmware releases with improved hotend temperature stabilization when that is the case.
Preheat error and Preheat error (Heatbed) happen before a print even starts, when the printer cannot get the hotend or bed up to temperature in the expected window. Same troubleshooting order as above: ambient temperature and drafts first, then confirm the thermistor and heater connectors are seated (check fuses too, a blown fuse presents the same symptom), then run the hairdryer test Prusa describes, blow hot air at the part in question and watch for any temperature movement on the LCD. No movement at all means the thermistor itself is dead. Movement, but the printer still won’t reach target, points to the heater cartridge instead.
Preheat the nozzle! is a much simpler message. It shows up if you try to load filament before the hotend has reached the minimum 190°C loading temperature. Preheat through the material menu or manually via Settings, wait for the number on the screen to stabilize, then load.
PINDA probe, bed leveling, and homing
The MK3S+ finds Z height and bed tilt with a PINDA inductive probe mounted next to the nozzle (Prusa specifies SuperPINDA as standard on the “+” revision, a version that does not need a separate ambient temperature compensation the way the older PINDA on MK2.5/S did). Most of what goes wrong here surfaces during the pre print “Calibrate Z” pass rather than mid print.
Bed leveling failed comes in three documented variants, and Prusa treats them as three different problems:
Sensor didn’t trigger. Debris on nozzle? Waiting for reset. Clear any plastic off the nozzle and bed, then rerun Calibrate Z from the LCD menu. If it keeps happening, run the full XYZ calibration again rather than just Z. Reported causes from Prusa’s own comment threads that hold up, excess friction on the X or Y axis keeping the extruder from reaching the sense point, or the PINDA having physically shifted after a collision (it should sit about 1mm above the nozzle tip).
Sensor triggered too high. Waiting for reset. Check that the hotend is fully seated in the extruder body first, then recalibrate Z the same way.
Sensor disconnected or cable broken. Waiting for reset. Confirm the PINDA connector is properly seated on the EinsyRAMBo, inspect the cable for zip ties pulled too tight, and test the sensor itself per Prusa’s PINDA/SuperPINDA sensor testing guide. Run the full calibration wizard again after any hardware adjustment.
The related XYZ calibration error messages appear during first time or repeat calibration rather than during normal printing: “Bed calibration point was not found,” “Left front/Right front/Front calibration point not reachable,” and the softer “XYZ calibration compromised” variants of the same. The compromised versions mean the printer finished calibrating but one of the reference points barely passed, the printer is usable but worth re checking. All of these point back to the same root causes as Bed leveling failed: a dirty nozzle, a PINDA that has moved, or a mechanical binding on an axis.
Fans
Err: EXTR. FAN ERROR means the small hotend side (Noctua) fan is not reporting RPM. Expected range is roughly 3500 to 4200 RPM at full speed, checkable from LCD Menu, Support, Extruder Info. Clear debris first (tweezers for anything large, compressed air for dust, holding the fan blades steady while you spray so you don’t stress the bearing), then if it is still misbehaving, check for a fan not fitted with RPM sensing (in which case fan check monitoring can be turned off in Settings) or a genuinely dead fan, which needs replacing.
Err: PRINT FAN ERROR is the same underlying check applied to the front facing part cooling fan. Same debris check first. If that does not clear it, Prusa’s guide has you run the fan manually at full speed from Settings and move the extruder along the X axis by hand while watching whether it stops spinning. A fault that only shows up under cable movement usually means a broken strand in the yellow signal wire near the extruder body rather than the fan itself.
Filament handling
Please open idler and remove filament manually shows up when the extruder motor cannot pull filament back out on its own. It is not a hardware fault by itself. First just try loading filament back into the preheated hotend, the tip often needs to re melt before it will retract cleanly. If that does not work, Prusa’s illustrated manual removal guide walks through opening the idler and pulling the filament out by hand. If this message loops on every attempt to load or unload rather than resolving after one manual removal, Prusa’s own support response points at the IR filament sensor (labeled “Fil. sensor” or “IR” in newer firmware) giving false readings, worth re seating that sensor in the print head and checking its live value from Support, Sensor info.
On the filament runout sensor specifically: Prusa does not give the IR sensor its own named on screen error the way MINTEMP or THERMAL RUNAWAY get one. Instead a misbehaving sensor shows up indirectly, false “insert filament” prompts, unexpected pauses, or the idler message above repeating. Prusa’s IR Filament Sensor Troubleshooting guide covers firmware version checks (the sensor needs the matching firmware to report correctly), wiring, and reseating the sensor cable as the main fixes, plus a note that on some printed extruder motor plate revisions the tolerances around the lever are tight enough that a little material needs shaving off the part for it to stop false triggering.
If you have the Multi Material Upgrade attached, its error messages live on their own knowledge base pages rather than inside the standard MK3S+ print error message index, MMU loading failed, Load to extruder failed, FINDA: Filament stuck, and Unload manually cover the most common jams and sensor faults. Prusa is explicit that MMU troubleshooting only makes sense once the base printer itself, PINDA, extruder, filament sensor, is confirmed working correctly, so it is worth ruling out the printer side causes above before assuming the MMU unit is at fault.
Power panic and print recovery
Blackout occurred. Recover print? appears after a longer power interruption cools the hotend and bed down mid print. Prusa’s guidance here is short: check that everything printed so far is still stuck to the steel sheet, follow the LCD prompt to resume, and then actually stay with the printer for about ten minutes to make sure the resumed layers are adhering properly. If part of the print looks like it is coming loose, Prusa’s advice is to restart the object rather than fight it.
Mainboard and drivers
TMC driver overtemp refers to the TMC stepper driver chips on the EinsyRAMBo board, near the X, Y, Z, and E motor connectors. Before assuming the board has failed, Prusa lists several things to rule out: check the onboard ambient temperature reading (Support, Temperatures, Ambient, it runs warmer than room temperature normally but should not sit above 60 to 70°C), check ventilation if the printer lives in an enclosure, look for dust or grease buildup on the drivers and clean with isopropyl alcohol if needed, reflash firmware in case of a one off firmware bug, and check for aftermarket firmware or custom G-code that pushes motor current past spec. A short circuit or voltage dip, often traced to the print fan’s connector or wiring, is also called out as a cause.
Print file and SD card
File incomplete. Continue anyway? is the printer flagging that it could not find the expected end of file G-code markers on a file read from SD card or USB. It is not a hardware fault. Prusa’s checklist: confirm the G-code exported completely (nothing cut off mid transfer), make sure the start/end G-code sections in PrusaSlicer were not stripped, always use Safely Remove before pulling the card, strip special characters or accented letters from the filename, and reformat or swap the SD card if the fault repeats across different files, which points at the card itself rather than any one G-code.
When to stop troubleshooting and contact Prusa support
Prusa’s own articles repeatedly hand off to support once the physical checks are exhausted, particularly for MAXTEMP, TMC driver overtemp, and preheat errors that persist after a thermistor and heater have both tested fine. Contact goes through the live chat or support email linked from shop.prusa3d.com, and Prusa’s staff responses in these threads consistently ask for a short video showing the exact temperature or fault behavior, plus firmware version, rather than a text description alone. If your printer is still under warranty, do not disassemble the hotend or mainboard beyond what these guides describe, since that can affect a warranty claim. Prusa’s warranty terms are on a separate knowledge base page and are worth checking before ordering replacement parts yourself.
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.