9900 can't print or nozzle check unless in service mode

Our 9900 has developed a strange issue. When I try to print a nozzle check (from the control panel on the printer), the print head moves back and forth (across the entire 44" width of the printer) and doesn’t print anything. It won’t stop moving back and forth unless I manually cancel the print. If I send a file to print, it also does the same thing (print head moves, nothing prints). I’m running a 9900 with the pro inks and printing from Print-Tool 2.1.1 on a mac.

However, if I boot into service mode, I can print just fine. Has anyone else experienced this? We haven’t made any changes to the machine since setting it up (no firmware updates or anything like that).