The CORE One calibration wizard is a sequence of separate tests. Its error screen tells you which system failed, so keep that exact wording.
On an assembled printer, the wizard checks fans, motion and other electronics, then guides the loadcell and filament-sensor procedures. During the loadcell test, open the door and wait for the printer to display Tap nozzle NOW. Tap the cool nozzle from below. Touching early or leaning on the machine can spoil the test.
Filament-sensor calibration begins with no filament in the extruder. Prepare a short straight piece and insert it into the side PTFE inlet only when prompted. The CORE One uses a side sensor as well as the Nextruder sensor, so a failure can come from either point in the path.
If a test fails, remove loose objects from the chamber, confirm the sheet and toolhead are installed correctly, and place the machine on a stable table. Rerun only the failed section from Control > Calibrations & tests after correcting a real cause.
Do not adjust CoreXY belt tension on a factory-assembled unit as a first response. Kit and conversion manuals contain belt steps because those machines were built by the owner. Use the support article that matches your printer form.
A passed self-test does not replace first-layer observation. It confirms that the machine can measure and move; the next short print checks the full workflow.
These tests sit within the broader calibration sequence outlined in a general calibration order for 3D printers, which explains why steps are normally run in this order rather than skipped around. Once self-test passes cleanly, the CORE One first print guide is the next practical check, since a passed wizard does not replace watching the first layer.