Why doesn’t my tank display doesn’t read full when the tank is full?

The resistive tank sensor should produce a resistance value of 33 ohms (American standard) or 180 ohms (European standard) when the tank is full. One possible reason that the tank doesn't register full might be that the sensor doesn't produce the correct resistance values. Another reason might be that the fluid doesn't doesn't go to the full extent of the sensor (e.g., a vent hole drains fluid before fluid reaches the top of the sensor). You may want to consider using the custom calibration feature of the TLA100 to configure the fluid level that you want to read as "full".