Load cell errors on MK4, MK4S and CORE One

Symptom

The screen shows a load cell error (the print-head sensor that detects when the nozzle touches the sheet) and the printer does not continue. For example «Loadcell Not Calibrated» (#13523 on MK4, #26523 on MK4S, #31523 on CORE One), «Loadcell Measure Failed» (#13526, #26526, #31526) or «Loadcell Bad Configuration» (#13527, #26527, #31527).

Likely causes, from most to least likely

1. The load cell calibration was not finished, or the printer did not register it as finished («Loadcell Not Calibrated»).
2. The load cell cable or connector on the print-head board (LoveBoard) is loose, disconnected or damaged, and the load cell returns an infinite or undefined value («Loadcell Measure Failed»).
3. The printer detects an incorrect load cell configuration, unrelated to tare or calibration («Loadcell Bad Configuration»).
4. The load cell is damaged (for example the white pad on top) or print-head bolts are too tight and distort the measurement.

Checks to tell which one it is

• Write down the name and exact number of the error: the last three digits give the type (523 not calibrated, 526 measure failed, 527 bad configuration) and the first two the model (13 MK4, 26 MK4S, 31 CORE One).
• Run the load cell test (Control → Calibrations & Tests → Loadcell Test) and watch whether the progress bar fills evenly when you tap the nozzle as the screen asks.
• With the printer off, unplugged and cold, check the load cell connector and cable, which plug into the right side of the LoveBoard.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Repeat the load cell test

   Unload the filament. Go to Control → Calibrations & Tests → Loadcell Test and follow the screen: tap the nozzle only when asked and do not handle anything else.

   Check: The bar fills evenly and the error does not come back.

   If that does not fix it: Check the load cell connection (next step).

2. Check the load cell cable and connector

   Turn off and unplug the printer and let the nozzle and bed cool down. Remove the cover on the right side of the LoveBoard (one M3x10 bolt on the MK4 and MK4S; two M3x6 bolts on the CORE One) and check that the load cell connector is fully seated and that the cable is not loose or damaged.

   Check: After reconnecting and reassembling, the load cell test passes.

   If that does not fix it: If the cable is damaged or the error remains, go to the next step.

3. Check the bolts and the load cell pad

   Following Prusa's load cell guide for your model, with the printer off and cold, slightly loosen the hotend fan bolts and, if that is not enough, the three heatsink bolts that hold the load cell. When you remove the print-head case, look at the white pad on top of the load cell without touching it.

   Check: The load cell test fills the bar evenly again.

   If that does not fix it: If the pad is damaged or the test still fails, the load cell is built into the heatsink and Prusa says to replace that assembly: contact their support.

Prevention

• After assembling or repairing the print head, complete the load cell test (Loadcell Test) before printing again.
• Do not overtighten the print-head bolts when reassembling it.

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Printers it applies to

Prusa MK4, MK4S and CORE One.

Official sources

This guide summarises the official documentation linked here:

• Loadcell Not Calibrated #31523 (CORE One) #26523 (MK4S) #13523 (MK4) · Prusa Research
• Loadcell Measure Failed #31526 (CORE One) #26526 (MK4S) #13526 (MK4) · Prusa Research
• Loadcell Bad Configuration #31527 (CORE One) #26527 (MK4S) #13527 (MK4) · Prusa Research
• Loadcell troubleshooting · Prusa Research