CHANGED TONER NOW A BLACK SMUDGE DOWN SIDE OF PAPER

If you use new original toner, this should not happen, nevertheless, clean the drum first.

this is problem with fuser teflon sleeve.you have to change the sleeve

If this smudge is showing up on the faxes you are RECEIVING and you have changed the toner then the next most likely cause is a fuser problem. This is especially true if the smudged area feels gritty or the toner rubs off the page in the smudged area. If this is the case then you need to replace the fusing unit or have a local repair place replace the fuser "film" on your fuser unit.
Good Luck and Thank You for using Fixya.

Problem is either the cartridge or the fuser.
Put paper in the manual feed tray(tray 1)
Print something. When the paper gets 1/2 way into the printer, open the
top cover. Pull out the toner, look down at the paper.
If it has the smudge replace the toner. If no smudge, replace the fuser.

Try Using the HP Print Diagnostic Utility in Windows - HP Customer Care (United States - English).

Try taking out your toners, and on the drum you will see a green button which you can slide from side to side (it's on each of the colored slots). Make sure you put the the white plastic cover up and then it will slide. I hope this will help you.

Generally speaking, any area that is darkened, is receiving less light. This can come from a lamp that is failing (similar to a fluorescent lamp used for lighting developing a dark end) or a mirror or optical lense that is contaminated with toner.
This may require some disassembly to diagnose and cure.
You may also want to visually check the fuser assembly (often accessible behind a door) to ensure there is no toner buildup on the affected side.

Hey, Pleas check Black imagine unit,if needed change it.

Its probably a defective toner. If the film on the toner drum is scratched it can cause toner to stick to the drum everytime it spins causing the constant marks.

Hmm... This sounds like it is a toner problem. If your roller is getting too much toner then it just globs it onto the paper even though the drum is not asking for it. Are you using a new toner cartridge or a re-filled one? Is it from Brother or a 3rd party? I suggest you remove the toner cartridge and check inside the printer. Clean any spilled toner. Check the toner cartridge for leaks .. give it a good shake and see if it is leaking. If you have another toner cartridge, you can replace it to test. You may need to pint a few pages to see a change. Another test could be to print a few (2-5) "BLANK" pages. The printer should not be depositing any toner. If no smudge or the smudge stays the same, then the toner cartridge is the likely problem. If the smudge is there, but gets smaller, then clean your printer.

Good luck!