you first need to establish through a wiring schematic if there is a separate sensor for low fuel or if the ecm reads a voltage return to indicate and calculate level and the alarm point, it could be the in tank sensor or wiring that is compromised so the voltage signal is out of whack, usually they are 2 wire connections, but some are three, and your dealer should have info of the resistance value from full to empty and the trip value for the alarm. sounds like the ecm overides a visual check and sets the code for engine protection/stupidity. what ever happened to common sense in design?