The magnetic Z stop sensor of my Printerbot quit working about a week after I first started printing. While I was waiting for a replacement printerboard, I leveled the bed using shims of paper and removed the auto leveling G code. I use a sheet of paper as gauge between the hot head and bed for the first layer. The prints started coming out better then when the Z stop worked. I did get the Z stop fixed, but I still prefer to print without autoleveling. I now only use the Z stop for homing.