1999 Buick Regal Fuel gauge

Typically the problem is with the fuel level sensor itself (inside the fuel tank) or the wiring to the fuel level sensor. To explain the fuel gauge's behavior the signal is being lost causing the fuel gauge to read empty or drop.
Are you mechanically inclined?
Thanks for the question.

Remove the fuel level sensor from the fuel tank. With the fuel level sensor out and still connected electrically. Examine the small fingers on the level sensor and make sure they are all there where it contacts the board. If the fingers are missing or bent replace the sensor itself if possible or the entire fuel pump assembly. Move the fuel level float and check the reading on the instrument cluster with the key on. Make sure to disable the fuel pump because it will turn on spraying fuel when you cycle the ignition key. With the fuel level sensor fully up the instrument cluster should read full and fully down should read empty. This gauge on the instrument cluster will move slowly so when you move the fuel level sensor give it some time before you move it again.
Reply if you have any questions