C-7980 means that the waste toner resevoir in the process unit is full with less than 100,000 copies on the unit. You can vacuum out the waste toner by removing the panel on the left or right side (can't remember). That should make the code go away.
if you get c7980 on a km-1500/fs-1018/1118 the process unit has more or less don its turn. it has happened on a couple machines we service, and they've been pretty close to 100000. change the p-u. (or sell them a new machine) check warranty btw.