I recently pulled out an older Samsung laser printer from storage, model CLP-315. It seems to work fine for the most part, except on every single page, it leaves a horizontal toner smudge. It even happens on the printer test page, or if I print blank sheets of paper from the computer.
If I print black and white, I get a black smudge. If I print in color, I get four matching smudges next to one another, in each toner color.
I've tried printing lots of blank pages to see if the smudge will decrease over time, but it does not seem to be fading at all. It does sometimes show up at a different vertical position on the page. I tried replacing the color toners and this did not change anything. I also wiped out the inside of the printer after taking out the toner cartidges, imager and waste container, but no improvement.
Here's what it looks like: