DIAGNOSIS The internal waste ink pad has reached its estimated capacity. This is a maintenance counter, not a physical sensor.
FIX The counter must be reset using a service tool. This requires a Windows PC, a USB cable, and the service software (often called Service Tool or SST). Download the correct tool for your model. Connect the printer via USB, run the tool, and select 'Maintenance' then 'Waste Ink Counter Reset'.
IF THAT DIDN'T WORK If the error returns quickly, the actual waste ink tank may be saturated. The physical tank is located inside the printer and may need to be replaced or drained by a technician.
HEADS UP Resetting the counter does not clean or replace the pad. If ink leaks internally, it can damage the printer. This is a known end-of-life maintenance alert for many Canon printers.