I have a HP laser 2600n and it is streaking magenta horizontally.

Vertical streaks:
Check,which color/s streaks are appearing on print out,remove that cartridge/s check on drum surface for streaks.. if found replace that cartridge/s..check.
Horizontal streaks:
This problem appears when the cartridge/s image drum not rotating properly. Resit the cartridges properly, print 20nos of blank pages, then check.... replace the cartridge..

are you sure the colour is not magenta,if so replace the magenta toner

check your magenta cartridge, and use a genuine toners

magenta toner is bad try a new one

I have the same streaks problem. Any solutions?

This happens due to Laser sCANNER Contamination.

Light Output that typically starts fading along the outside edges of the media. The first noticable color would be Magenta.

Dusty Environments can lead to Laser Scanner Contamination. The dust layer on the mirrors prevents proper transmission of the laser output in the imaging system leading to faded colors on the printed page. The initial stages of this failure typically occur in the magenta or yellow color planes as faded or light color. Often the left side is more faded than the middle.

You may need to contact the nearest Authorized Service Tech and get teh Laser Scanner replaced with the new released service part that has a mirror in a location where the dust ideally cannot reach eazily.

Also, please ensure that the printer is relocated from the current location to a more dust free zone.

You need to clean all the "widows" inside the laser unit too. Cleaning only the mirrors will not do any good. You can use some Methylated Spirits to clean it propperly.

51.22 is a Magenta laser scanner error.
Try removing the magenta restarting the printer and reinstalling.
If that does not work you likely need a laser assm.
You might want to call a tech. to replace.

Replace the Magenta Toner cartridge.

This is caused by toner contamination in the laser scanner unit, it is quite a complex job removing and cleaning the laser scanner. I would suggest you contact HP regarding the repair.