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] Press the Menu – Scan – Copy – Scan buttons, in that order. You will now be in service mode
2] Press the Left or Right buttons until NVRAM is displayed
3] Set the ABS-M level to 0
4] Press OK