On the Original Prusa XL, the selftest checks for issues in the assembly of the printer, its wiring and the electronic components. It is the first part of the Calibration Wizard, which is prompted automatically when you turn on the printer for the first time or after a Factory reset (XL). After the initial calibration is completed, the selftest can be initiated in LCD Menu - Calibration - Selftest.
During the selftest, its results are shown on the screen, indicating success or fail for each point. Alterantively, the results can be shown at any time on the following path: LCD Menu - Calibrate - Diagnostics - Show selftest results.
The Original Prusa XL selftest procedure can determine which of the fans doesn't work. Check the one that is diagnosed as not OK.
The printer can determine if the components are moving correctly along each axis.
In case of an issue on the X-axis, or on the Y-axis, make sure that there is no obstacle to the movement, and readjust the belt tension.
The selftest can determine a fault during the heating of a component, and the affected component.
Heat up the affected component in [LCD Menu - Settings - Preheat], and see if any error appears: if it does, the error should have a QR code that redirects to the article that describes how to troubleshoot that particular error.
A functioning loadcell is fundamental for the Original Prusa XL to detect that the nozzle is close enough to the steel sheet for printing, and to avoid damage that would be caused by the nozzle digging into the steel sheet.
The Original Prusa XL is equipped with two filament sensors: the first one is located on the side, close to the input of the PTFE tube where you feed the filament. The second one is located in the extruder. Both filament sensors are necessary for the correct filament retraction: when the sensor on the side detects that the filament has run out, the filament will be retracted in time.