What could be the causes that the fuel gauge is not working properly?

Hello, The fuel gauge is controlled by the fuel level sending unit in the fuel tank of both vehicles. The Cutlass will have a fuel pump with the sending unit attached, and the Malibu may or may not have the fuel pump attached. By checking the outputs of the fuel level sending units while moving the fuel level arm back and forth very slowly, the unit can be tested. Normally this is the issue, however, that is not a guarantee, as both gauges could be bad, or wiring could be faulty. If you always use the same fuel station, they may have too high of sulfur in their fuel, causing the fuel level sending unit to become fouled.