ImageCLASS D1550 prints black bands on entire page

[CommercialGC]

Hello! We have the ImageCLASS D1550 printer in our office. It's a small business with a small staff, and documents are what we use this printer/copier for. We have been having an ongoing and worsening issue with the printed product. Over the course of 3-4 weeks, what started out as a few faint horizontal lines on the page has turned into many black bands of little lines, making it virtually impossible to even read what was printed or copied. 

This happens when we print document files, image files, webpages. This happens when we copy documents or images on the glass. This happens when we copy documents or images with the document feeder on the top. The machine does not discriminate on source or format. 

We all print wirelessly. We use regular white copy/print paper. We have tried cleaning the little clear strip on the left side of the document glass. We have tried replacing the toner, and yes, it was Canon brand. The problem continued to worsen. I also tried going into adjustment/maintenance >> clean fixing assembly >> print cleaning sheet, and then inserting cleaning sheet printed side down into the multi purpose feeder in the front of the machine. I did that 5 times. I can see a very slight improvement on the printed sheet, meaning there are fewer horizontal bands. When I do a test copy of a document, the product output looks the same, without comparable improvement compared to the printed cleaning sheet. 

Why is this happening? What else can be done to resolve the issue? Thanks for your help. 

[CommercialGC · Accepted solution]

Hi, Original Poster here. Came back to say that out of desperation once our printouts reached the point of being unreadable due to black horizontal lines across the entire page, I called Canon Tech Support. I showed them these images, and explained  corrective measures already attempted. Images were then shown to one of their engineers, and the verdict was: due to the face that the lines are horizontal and evenly spaced, and that the print quality is getting worse, it's most likely that our toner cartridge was leaking. They said to switch to any brand other than whatever was currently in place and try printing/copying with the new toner cartridge. I bought a Canon brand toner just to be safe, and sure enough, the problem was solved. Leaking toner cartridge was the culprit. Now you know!