How are fuel level sending unit issues diagnosed?
Suspected fuel level sending units are normally a very quick diagnosis. The servicing technician will immediately test the system, and verify that the gauge is working properly when the proper conditions are applied to the circuit. If the gauge and all fuses are in working order, the voltage from the fuel sending unit will be compared to specifications, and electrical connections will be verified. The readings from the sensor will indicate if failure has occurred once the remainder of the system is verified in good working order.

How is a fuel level sending unit replaced?
The fuel level sending unit must be removed from the fuel tank. In most vehicles, there is a service access panel underneath the trunk linking, or back seat, however, this is not always the case. Some vehicles require the fuel tank to be removed in order to access the fuel sending unit. Often, the sending unit and fuel pump must be replaced together. They are unbolted from the fuel tank, and disconnected from fuel and electrical connectors. Note, the technician will disconnect the battery prior to accessing the fuel pump. The new pump is inserted, connected, and tested prior to reassembly of the vehicle.
RepairPal Recommendations for fuel level sending unit issues
Prior to installing the fuel level sending unit into the fuel tank, we recommend comparing the movement of the old to the movement of the new fuel level sending unit. If one moves farther in either direction, the arm should be adjusted to ensure the proper level will display on the gauge.
What to look out for when dealing with fuel level sending unit issues