I'm having the same problem, but without a 5200 error. My PIXMA G6020 was printing just fine.

Now it acts as if it were a "black ink only" printer. The diagnostic prints from ALL maintenance routines
(head alignment, cleaning, deep cleaning, etc.) show only black ink results, as if the color print heads don't even exist!
The diagnostic print outs *** used to *** show results in black, yellow, cyan, and magenta... now only black.

When performing a color copy from the printer's scan platter, it prints in black ink only, printing the image in grayscale,
as if I had black copy button. When I print a color image from my Windows 10 laptop using Photoshop,
the printout shows only the parts of the picture where the black ink head was used. The color portions of the image are missing
from the printout completely. When I printed the same color image with the "grayscale printing" option checked
in the advanced print settings dialog, the entire image printed, but in grayscale only
(i.e. a black and white picture...as if I printed a color image to a printer that only prints in black ink).

Since the problem appeared suddenly, and similar behavior occurs from the printer itself and print jobs spooled from
a computer, it seems like this could be a firmware issue. I vaguely remember doing a firmware update
(this was week's ago), but I don't remember. Will post a reply once I get this figured out.