Repetitive defect rulers help determine what component is causing the repetitive image defect. You simply measure the distance between defects and look up the corresponding component. That said, you really don’t need these rulers. Repetitive defects are almost always caused by the toner cartridge or fusing assembly, and occasionally the transfer roller. If you have an extra toner cartridge or fusing assembly, swap them to see if the problem goes away. Otherwise you can do a half-way test.
A half-way test is a simple test to determine where the print defect is occurring in the printer. As paper travels through the printer, it first passes under the toner cartridge and then into the fusing assembly. Color printers are a bit more complex but the concept remains the same. The idea is to interrupt printing before the paper gets in the fuser. What you do is print a page and open the toner lid when you think the paper is underneath the toner cartridge. This will stop printing and you can pull the toner cartridge out and look at the partially printed paper. It may take a few tries to time it so the paper stops under the toner but not completely into the fuser. If the repetitive print defect is on the page before the paper reaches the fuser, then the fuser is not the problem. It’s probably your toner cartridge or maybe the transfer roller.
Another simple check is to remove the fuser and look at the long rollers in the fuser. If you see toner baked on the rollers or any other gunk then you need to replace the fuser. Fusers with plastic fixing films, also called sleeves, can tear. This usually causes a smear of toner all the way down the page where the sleeve is ripped.
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