Single Black Horizontal Line on LaserJet M751

[shulerd]

I have a Laserjet M751, running from my Mac (OS 10.14), updated firmware and drivers. I have a single, fine,  black horizontal line running across every page right now.  I've run the calibration and cleaning to no avail.  I printed the Print Quality diagnostic pages and this line appears on each page.  I've removed the fuser and did not notice any damage.  I removed the transfer unit and did see a streak along it, though it wiped off with a cloth.  There is also a black ring on the black toner cartridge.  Since the fuser, transfer unit, and a new cartridge appear to cost $900, I'm hoping to further troubleshoot this before ordering expensive replacement parts since this is merely a fine black line on every page.

[Repairatrooper]

If you have a mark on the cartridge that is the first place I would start. The black cartridge is always in contact with the belt so if it is bad it can taint all test prints. What is the page count? It could also be a overfilled wastebin on the transfer belt that has failed to drain into the waste toner bottle. You can test this by removing the belt, and over a garbage can, open the shutter on the front right of the belt to see if toner drains out.

[ScannerAndPrint]

Since letter/A4 size paper can be loaded short edge first of long edge first, it's hard to know what is horizontal.  Can you attach a picture?

Try re-seating the drums. If it's merely a poor connection that could help. If not, you can call support and you should be covered under warranty for any service parts.