The level indicators on black and grey tanks are notoriously incorrect. The system consists of 3-4 metal sensors ( many times just screws) in specific positions in the tank. When the liquid in the tank reaches a sensor it conducts electricity back to the display panel. If a piece of crud ( food, paper, etc) gets stuck on the sensor, it conducts to the damp sides of the tank and appears to be a liquid level. Deciding when to empty your tanks is a combination of indicator readings and understanding roughly how much liquid was put in the tank. Sometimes cleaning the tank works and sometimes it doesn't.
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