Changing the toner cartridge for magenta may indeed fix the problem, however before you try that since it costs about $150 for the cartridge how about some more information. How old is the printer and what is the page count total for the printer. Were there any jams, use of heavier than normal paper that preceeded the red line appearing? Additionally you can try to clean the laser lens on the cartridge using the laser lens wand that came with the printer. If the printer is old it could be that the transfer belt isn't being cleaned properly by the transfer roller.

Have you tried changing the printer cartridge? The software of your printer has it's utility to clean / allign the cartridges for better printing quality.