If you are experiencing lines down the prints from your laser printer, chances are they will look something like those shown on the image below. Whilst it is the black shown here you may experience cyan, magenta or yellow lines from colour laser printers. The position, thickness and definition may change but the cause will remain the same.
What causes these lines?
Distinct lines of this nature are usually caused by a defect of the colours or drum roller within the cartridge or a drum unit/photoconductor unit. The defect can be anything from a physical dent in the roller to a manufacturing defect that is preventing even distribution of the toner powder.
What can be done to resolve this?
Sadly there is not much that can be done as an end user to fix the issue with the existing set of components installed. What is required is the identification and the replacing of the defective component. Unless you noticed the lines appear immediately after changing a certain drum or toner cartridge, this may be easier said than done. With the cost of replacement toner cartridges and photoconductor units I am sure you will be keen to get it right the first time.
How can I identify the defective component?
As mentioned previously, problems of this nature are generally caused by the roller of the photoconductor. As such we need to work out where the photoconductor lies in your particular machine. If your printer takes separate drum units and toner cartridges then it is the drum unit that would need replacing. If no separate drum unit is available for your printer it is likely that the toner cartridge has the photoconductor as part of the cartridge. If you are unsure simply look for a shiny roller, either clearly visible on you toner cartridge or tucked away behind a protective flap. Please see the image below and the green photocusductor unit roller clearly visible.
If such a roller is visible on the appropriate colour cartridge for your printer I would recommend replacing this unit. It is even possible that you will be able to see the defect if you turn the roller gently.
I hope this guide has helped resolve any print quality issues you have experienced. If however you have any other questions or would like clarification of any points please let us know in the comments below.