We had a machine that would work well for a few hundred copies after replacing the toner cartridge and then the printed image would fade. Being lightest in the center and almost normal density at the edges. At first we thought it was defective toners, but it turned out to be the laser unit. We have seen this on 2 units after about 8 months - 1 year of use and both have been fixed by replacing the LSU.

It is the laser LSU, also there can be a Power supply failure that occurs too, bank of resisters over heat.
but the faded is LSU.
there is a Firmware update if the light copies are all over even. Do the formware then LSU