I was printing something in colour when the printer told me that the yellow toner is low, and suggested I replace it.

I had purchase a full set of spare toners late last year for just this occasion so I was prepared. I popped in the yellow toner, and pressed continue ... and this happened:

There were repeated horizontal lines across the paper.

Now the toner I used was a "compatible" toner, so my immediate thought was "serves you right".

My next thoughts were trying to figure out which online seller I'd bought the toner from and whether I still had a copy of the receipt. Fortunately, I did, and emailed them about the yellow toner being faulty and got a reply within a few hours to send it back to them, and they'll send a replacement immediately.

So I popped out the yellow toner and put back the old one ... tried a print and eh ... the problem was still there. Oh no ... it's damaged the printer itself. Darn ... went to sleep pretty geram.

The next day in a cooler state of mind I attacked the problem again. This video explains that a repeating band is likely a defect on one of rollers in the printer, with the distance between the defects equal to the circumference of the roller (I measured the distance at 63mm ( c = πd ) which works out to 20 mm). Typically it's the photosensitive drum.

This printer has a drum integrated into the toner cartridge, so replacing the cartridge should solve the problem. So why did the problem persist when I reverted back to the old yellow toner cartridge?

I fired up inkscape and printed out each of the C, M, Y and K colours on separate sheets of paper.

And voila ... the culprit is revealed! It's not the new yellow toner I had just put in, but the cyan toner that has a problem.

Why did it only happen after I changed the yellow toner?

Here's where human factors cloud troubleshooting. Thinking it through, it did not "just happen after I changed the yellow toner" but I just happened to notice it after changing the yellow toner. According to the printer, I changed the cyan toner on April 10th, and it's quite likely I haven't printed anything that exposed the defect since that time.

Upon close inspection you can see the band on the cyan toner's drum ...

So the cyan toner gets sent back tomorrow. The printer is sufficiently smart stupid that it won't print without a missing toner cartridge (and you can't fool it by putting a different colour cartridge in the cyan slot) ... so hopefully I get the replacement soon.