Start with basics.
Power off, disconnect the center wire to the float sensor. Put an ohmmeter on the center screw, and the other lead on ground. If the tank is empty, it should read about 30 ohms; if the tank is full, about 230 ohms.
Make sure the sensor has a good ground connection, NOT to one of its mounting screws (these get covered in sealant and sometimes make a poor electrical connection.)
If it reads 30 ohms when full the sensor may be stuck. Some people have rapped the mounting plate or tank bottom with their hand, and had it come unstuck!
Next, re-connect the center wire and turn on aircraft power. Put your meter on volts, measure center connector to ground. You should see a very small voltage if empty, and about 2 volts if the tank is full.
Repeat with the other tank.
If it passes these tests, then it?s the software set up.