Two black marks on the back of the page HP LaserJet P3015
Is the toner defective or did i do anything wrong?
If you are consistently seeing 2 small marks on the backside of printouts from your HP LaserJet P3015 series printer, you probably need to replace your registration assembly. There is a section of the plastic guide plate that tends to wear out over time. That worn-out plastic guide plate catches the leading of the paper, and causes the feed rollers to leave 2 small marks on the dragging page.
The original design of this assembly from HP had different-angled plastic and does not hold up very well.
Replacing the registration assembly is not considered difficult, you can search online for instructions.