Without warning I also got the dreaded B200 error. These are the steps I followed, and successfully resolved the problem with B200 error.
1. You may want to search and watch a YouTube video on how to remove the print head from the MX900 series. If not, this is a summary:
2. The print head is buried behind the 5 ink cartridges that need to be temporarily removed for access to the head.
3. In front of the ink cartridge bay is a plastic bar labeled with the ink color slots. You will need to slide the ink bay to right enough where you can pull the ink bar forward about 1/2", and lock open. This step releases the print head.
4. Reach to the back and grab the print head by the center tab, which pulls the whole unit out. Don't force anything, either slightly lift up or rotate forward and it comes out without effort.
5. Clean the print head. I used rubbing alcohol but others have posted recipes for different mixes, or vendor solutions. Some have soaked the head for a few hours in a cleaning solution. Dry the head.
6. Reassemble. With the ink bar locked forward, hold the print head unit by the tab and gold electrical contacts facing up.
7. Use a flashlight so you can see what you are doing. Slide print head unit toward the back. With both sides even & level, push the top and the print head unit will rotate forward and snap into place with little effort. The electrical contacts on the print head will align with contacts on the back of the cartridge bay.
