Holding tank sensor reading wrong (stuck full)

Tank sensors reading full when the tank's empty (or the reverse) is almost always buildup on the sensor pads themselves, not a wiring fault. The sensor pads read capacitance/resistance through the tank wall, and toilet paper residue or waste buildup coats them and throws off the reading permanently until cleaned. A tank cleaning wand or a few flushes with a proper tank cleaner (not just water) usually restores accurate readings. If you've cleaned it and it's still wrong, then it's worth checking the sensor wiring at the monitor panel. Which tank, and has it ever read correctly?