What causes the laser printer to print all pages with a washed out background?

To understand this better, you may want to read about how laser printing works.

A gray cast indicates that the drum is not getting or holding a sufficiently strong charge to hold the toner. It looks like either the OPC drum or the corona wire/primary charge roller is not working correctly. Try replacing the cartridge (the drum is part of the cartridge in many HP laser printers, including yours). If this does not work, the printer needs to be serviced.

This answer is almost 7 years too late, but I believe it may be the definitive answer to the washed-out prints on the LaserJet 1200/1300 printers.

I recently spent several months trying to find the cause of this problem on my LaserJet 1300 (nearly identical to your LaserJet 1200). I too, had been using an off-brand toner cartridge (Cartridge World), and at some point it stopped being recognized by the printer. After replacing the cartridge, I had prints, but they were faded so much that the pages could almost be mistaken for blank.

I tried everything mentioned in the other comments and answers, and spent hours combing the internet for information on this problem. I cleaned every accessible part, including the corona wires, and replaced the toner cartridge with a brand new HP cartridge. I even took the laser off and cleaned the mirror (unnecessary). Some references online mention that the photo-sensitive roller in the cartridge can go bad on the shelf (it didn't, I tried a second brand new HP toner cartridge with the same result).

Finally, I disassembled the entire printer piece by piece because I was determined to save it (after 16 years without a single problem, I had some loyalty to this thing).

In my case, the problem was a faulty zEngine Controller Board - it destroyed a toner cartridge in the process (or perhaps the cartridge itself developed a fault that shorted out the board). I replaced the controller board with a used one from eBay for about $19. As soon I removed the original board, I could see a row of resistors with burn marks (R540-R544 next to the large capacitor).

The zEngine Controller Board is accessible from the back of the printer:

The area of the fault on the board:

A closer look at the blown resistors:

And here are the test pages after replacing the board:

Replacing the board was pretty easy, but I wouldn't call it trivial. It took about three hours even with YouTube videos showing the entire process (this video about removing the fuser will get you most of the way there). The good news is that it only took patience and a couple of decent Phillips screwdrivers (two sizes).

In retrospect, the source of the fault may well have been the toner cartridge, because both that and the board needed to be replaced in order get a functioning printer again. Additionally, the off-brand toner replacement leaked toner into some hard-to-clean areas in the printer. From now on, I'm spending the extra money on the HP cartridges.