Hello mahlonheller,
There should be either a Fault Code or Message and the message should have either a letter C or M or Y or K at the beginning of the "Toner Needs Replacing" message to tell you which one needs to be changed.
C = Cyan
M = Magenta
Y = Yellow
K = Black
How to determine which toner cartridge is low in toner? I am getting notice of low toner but not which one/