The Prusa i3 tossed up one of its first calibration errors for me today;

Bed leveling failed. Sensor triggered too high. Waiting for reset.

That seems bad. A bit of googling, and a quick trip to the Prusa Forums
pointed to something being mis-aligned between the Z axis heights stored to the printers EEPROM during calibration at the factory, and the bed height currently detected by the PINDA probe during pre-print calibration. Which of the 9 calibration points it happens at seems to vary for people – for me it was calibration point 2.

Toggling the power to the printer to reset didn’t solve the problem – the pre-print
calibration would always fail at the 2nd calibration point. A careful read of the forums and user manual indicated that re-running the full XYZ calibration was probably the best option; at some point the printer got jostled, or the surface under the printer warped, and the current print bed height was too different from the stored calibration height for at least one of the
calibration points.

I unloaded the currently loaded filament from the printer before running the full XYZ calibration. The second or third step of full calibration has the printer mapping 4 points on the print bed to establish the XY axis. During this phase the manual advises that you place a standard sheet of paper on the bed to track whether the print nozzle is impacting the bed (that would be bad – and the manual helpfully suggests that you should immediately power-down the printer if you notice the sheet of paper getting caught and dragged by the print nozzle). I had problems with this step; the first three times I tried the nozzle would catch the sheet of paper.

Before the fourth try at recalibration I really carefully inspected and cleaned the print nozzle, and noticed as I did that about 0.5mm of filament had been poking out from the nozzle. Trimming off that filament and cleaning the nozzle seemd to do the trick: the fourth recalibration went flawlessly.

The last step of recalibration, as suggested by a message from the printer on the control LCD and in the Prusa manual, is printing the V2Calibration.gcode model. Printing the model only takes two minutes, but provides an easy way to validate that the Z axis is properly calibrated to get the print nozzle to the right height for printing. If the nozzle is too high, filament won’t adhere, and you’ll just drag bundles of filament around the print bed. If the nozzle is too low, you’ll flatten out the filament too much as it prints, or worse, impact the print bed with the nozzle. I used the Live Adjust Z controls of the printer to fine tune the Z axis as much as I could to get a clean print of V2Calibration.gcode. Given how quick the model prints, it took 3 consecutive prints to get things adjust to the point where the model was printing smoothly and evenly.

Hoping that this had been enough, I jumped back to the model I had been attempting to print when the “Bed leveling failed. Sensor triggered too high. Waiting for reset.” error popped up, but I couldn’t get a print to successfully lay down the first layer. More Live Adjust Z tweaking resulted in an overall Z axis offset of about 0.425mm. After that, and a bit of glue, I got the original model I had been attempting to print off to a good start.