In this printer the waste toner or toner not transferred is then taken back into the print cartridges waste toner collection section. A print cartridge that is leaking toner back onto the belt can cause toner to get back onto the belt and then onto the back of the print media. The belt should be clean after a print cycle. If you see toner on the belt it is most likely coming from leaking or defective toner cartridge and should be replaced. A visual inspection of each print cartridges drum might be all that is needed. The drums should be clean after a print cycle with no visible toner on them when the printers front door is opened and the belt is released from the locked position. If there is toner on the print cartridges drum, it should be replaced.