: Power the unit on and preheat both the nozzle and the bed. Use the printer's display to verify they reach and maintain target temperatures without fluctuating wildly, which could indicate a failing thermistor or heater cartridge.
: Shake the print head and bed gently. Any play or "slop" indicates loose eccentric nuts or worn-out wheels that need immediate adjustment or replacement. buying a used 3d printer
: Use Digital Calipers to check if a 20mm test cube is actually 20mm. Major deviations suggest worn-out belts or incorrectly calibrated E-steps. : Power the unit on and preheat both the nozzle and the bed