I was worried that my used 3D printer might not have been maintained well before I got it, so I decided to give it a bit of a cleaning. I used a copper-bristled brush to give the extruder nozzle and alignment sensor a good cleaning. Then, I wiped down all the rods inside and applied some white lithium grease to them and to the z-axis bolt thing. I also used a cotton swab to apply some of the grease to as many of the pullies as I could reach. The next time I tried to print, it failed (before starting) with a z-axis error. I was able to use jog mode to move the z-axis up and down, so I’m not sure what had gone wrong. After restarting the printer, my next print worked fine.