I have small black dots down the left hand margin approx 10cm apart- machine is only 12 months old

10cm means it's caused by a large roller. The only roller large enough for that would be in the drum. Replace your drum and it should clear up, the model number of the drum is DR520.

Check the drum unit. Rotate the optical cylinder slowly, looking for a scratch or clot of toner on the drum. You may need to clean it with compressed air or VERY gently with alcohol and a cotton ball. If the drum cylinder is clear of defects, check the fuser at the back of the machine for the same. A drum can be cleaned or replaced, a trashed fuser is not repairable. It may also be cheaper to replace the machine than to replace the drum of course.

The carrier slides on a silver shaft that is about 1/2" thick spray some WD40 on the shaft, if that doesn't do it you have to get some on the carrier motor bearings located under the case in the R. rear corner.

DickyO

Spots along left margin spaced vertically about 3 inches apart:

Brother HL-4040CN Spots

It's probably the toner cartridge again. See if you can spot defects at the cyan drum unit that matches the marks on your printout.

it sounds like the CYAN toner cartridge is leaking.

your dmd chip is defect this a good job you must tack emgine apart calculate atleast 200 dollars

try to align the printer with the printer settings

inspect the pc drum, if there is no dots, then its ok, also inspect fusing unit rollers and transfer belt for dots and clean laser glass.

do this procedure
1.print test page then stop the printer on the middle print proses by turn the printer of or open the rear cover
2.remove the paper jam carefully
3.look to the paper if the problem found change the drum unit els the fusser unit cause the problem