The selftest verifies the most important components of the printer. It is the first part of the wizard, which will start when you turn on the printer for the first time, or you can start it from LCD Menu -> Calibration -> Selftest.
Each part of the test can also be run individually from LCD Menu -> Calibration -> Test FANs / Test XYZ-axis / Test heaters. You can always check the results of the previously completed selftest from LCD Menu - Calibration - Show selftest results.
Pay close attention to both the printer and the LCD display. A green checkmark (✔) should appear by the part being tested. If all is green you are good to go.
With the exception of the heaters, a red X will appear next to the tested component if it fails. Make a note of where it failed and let the self-test complete. If anything does not check out, it will produce the message:
"The self-test failed to finish. Double-check the printer's wiring and axes".
Verify if there is nothing blocking the rotation of the fans - Debris might get between the blades. Bigger parts can be cleared with tweezers, and dust can be cleared with a computer cleaner spray.
In case of issues with either heater (bed/hotend), a red error screen will fill the LCD display, showing the error and a QR-code which will link you to an article for troubleshooting the component. In case this happens, check the wiring on the Buddy board.
Open the electronics box by the screw in the printed cable cover. Check if all of the cables are well connected. In case you find any loose connections, plug it back in and make sure you feel/hear the safety-latch lock into place. After that, close the box, turn the printer back on, and restart the Selftest or Wizard in the Calibration Menu.
Starting from firmware version 4.3.2, if you have a USB drive (FAT32) connected to the MINI/+ when performing the Selftest, the printer will save three logfiles into the drive as a text file. In the files, you can see the results for the tests on the two fans, the two heaters, and the three axes.
These files might be used for troubleshooting any selftest issues which have not been resolved through normal troubleshooting.