Every time your printer runs a head cleaning routine, it takes ink from the cartridge, and forces it through the print head in order to try and clear any blockages. This ink ends up in the waste ink tank.
After you have printed a certain number of pages, the printer will automatically say that the waste ink tank is full, invariably though, it isn't anywhere close to being full.
You can reset this page counter by following the instructions below.
Please note, you do this at your own risk, if your waste tank is nearly full, you could soon have a lot of ink all over your desk!
1] Printer on
2] Press the Additional Functions, Scan, Copy, Scan buttons in that order. You should now be in service mode
3] Select TEST MODE
4] Select  PRINTER TEST in TEST MODE
5] Press the +/- buttons until 8-3 is displayed
6] Press the Select button The menu will display EEPRON Clear
7] Select Ink count which is option (0) and press Set button
8] Press the Stop/Reset button then the Power button.