Pale smudges regularly spaced down left margin, 1.5 inches apart

Hi:

Since you have already replace the toner cartridge then the most likely culprit is the fuser assy. You can test this by trying to print something and stop it by opening the cover just as the paper is trying to pass under the toner cart. Remove the toner cart carefully and look at the unfused print on the paper. Do you see the faint smudges? If not, then the problem lies with the fuser assy. This is the unit that gets hot and fuses the toner to the paper. If this is the case you will need to have a tech replace it for you.

Good Luck

Andrew